Comparing The Challengers

[Overmod]

I'm three blocks from all those Amtrak trains in one direction, and all those Brightline trains and FEC in the other, but aside from grade-crossing encounters I haven't had time to watch anything!

Those GO Transit cars on Tri-Rail make me nostalgic, though.  They were modern in the '60s and are still modern today.

[Overmod]

I'm three blocks from all those Amtrak trains in one direction, and all those Brightline trains and FEC in the other, but aside from grade-crossing encounters I haven't had time to watch anything!

[Erie Limited]

If you have to be stuck, what better place than West Palm Beach? OK, maybe Buffalo or Cleveland when the Indians are in town...

[Overmod]

I'm stuck in West Palm Beach with nothing but an iPhone, until at least Friday.

Guiding dynamics of a cab-forward are radically different from those of a conventional Mallet with a lead truck steering the hinged forward engine.

68" on the Big Boys is a function of lightweight rod work and modern balancing.  We might recall some of the early-2000s Kalmbach articles about practical augment reduction with smaller drivers, and the British 9F reaching over 90mph on what I recall to be 56" spoked drivers, with a 2-wheel lead and no trailing truck.

I have always read that the "80mph" in relation to Wahsatch engines is the balance speed, related to augment and only indirectly to guiding.  (By comparison, Glaze balanced the J to something like 510rpm for 'dash' capability of only 100mph).

[Pneudyne]

No problem!  In any event, locomotive design was a series of trades-off, with different roads and builders putting different weightings on the various elements.  Although no doubt some errors were made, when it comes to post-facto analysis – particularly of the non-professional variety, as certainly applies in my case - it is probably better to start with the assumption that the roads and builders reasonably knew what they wanted, knew which pathways were situationally acceptable and which were not, and then proceeded accordingly.  And then on that basis, attempt to impute reason to their choices.

 

Re the 2-8-8-4, all examples had drivers in the 63 to 64 inch range, as compared with the 68 inches of the Big Boy.  That I think was to provide for relatively fast running over the flattish/undulating Evanston to Green River section.  For the Ogden to Evanston run up the Wasatch alone, a 2-8-8-4 probably would have worked.  But the UP wanted the longer run for better locomotive utilization, with the eastbound freights handed over to the Challengers at Green River rather than at Evanston.  Thus, the Big Boys had to match Challenger speed east of Evanston.  Why 68 inch drivers rather than the 69 inches of the Challenger is unrecorded, but I suspect that the larger boiler size required a small decrement in driver size so that the locomotive would stay within the height limit.  In perspective, the Big Boy, with a fleet size of 25, was a special-purpose machine amongst the 105-strong general-purpose Challenger fleet.  An interesting statistic is that the UP Challenger fleet was quite a bit larger than any US road’s 4-8-4 fleet.  In a broad brush way, one could say that the Challenger was doing the same job west of the Mississippi-Missouri that was often assigned to 4-8-4s (mostly in the 69 to 75 inch driver class) east of the river.

 

If one includes the SP cab-forward 4-8-8-2 in the 2-8-8-4 group, then SP got there first.  Its initial use of a four-wheel truck under the cab may have been less to accommodate an enlarged firebox than to provide better guidance, the cab-forward 2-8-8-2 having proved unsatisfactory in that regard.  Given that the pilot truck was working on a relatively short moment arm when considered against the centre of mass that it was guiding, that was not so surprising.  But the design grew in weight to the point where the four-wheel truck was needed for weight-carrying purpose, and that was carried over to SP’s conventional 2-8-8-4.  The NP 2-8-8-4 was more-or-less the DRGW 2-8-8-2 with a much larger firebox as required to burn very low grade coal.  The DMIR 2-8-8-4 was derived from the WP 2-8-8-2, with the four-wheel trailing truck required to accommodate weight growth.  It had an incrementally longer firebox, but a smaller grate area of 125 ft² as compared with the notional 145 ft² of the (oil-burning) WP locomotive.  But according to the “rules”, a four-wheel trailing truck is sine qua non for entry to the “Superpower” category, so the DMIR 2-8-8-4 is admitted, whereas the WP 2-8-8-2 is not.

 

The B&O 2-8-8-4, with a nominal driving axle load of 60 000 lb (as compared with 70 000 lb +/- of the others) could be viewed as a “light” version.(relatively speaking) of the wheel arrangement.  It was the only example to be fitted with a single-plane articulation joint.

 

 

Cheers,

[daveklepper]

Thanks for the analysis.  Makes good sense.   The advanyages of the 2-wheel pilot truck are:  identical frong and rear "engines" for articulateds, and facor-of-adhesion being a greater percerntage of total engine weight.  The several advantages of the 4-wheel that you pointed ot were more important for the UP, and for other Railroads with 4-6-6-4s, as well, but the 2-8-8-4 was more popular than the 4-8-8-4.

[Pneudyne]

There was no doubt an element of tradition, so that it was unlikely that the Big Boy would have had other than a four-wheel pilot truck.  But there were other considerations.  One would have been weight.  At the time, the UP had a relatively conservative axle loading limit of 67 500 lb, and it was unlikely that the desired locomotive could have been realized as a 2-8-8-4 within this limit.  Thus, an additional carrying axle was required, which indicated a 4-8-8-4 or perhaps a 2-8-8-6.  Another factor would have been lateral railhead forces, which for a given set of stability and curving assistance requirements, would have been a lot lower with a four-wheel pilot truck than with the two-wheel type, however well the latter was executed.  Quite possibly the UP was as conservative with this parameter as it was with axle loading.  Given the ambient conditions in which it operated, low temperature cracking of rails might have been a concern.  Thus the 4-8-8-4 wheel arrangement both provided the requisite number of axles and kept the lateral railhead forces within the desired limits.

 

The Big Boy had the Alco-Blunt lever system of lateral controls, which can be viewed as a means of lateral flange force equalization across all wheels, perhaps the horizontal counterpart to the usual vertical equalization system.  This was nicely described in US patent 2230209.  One element was the use of pilot and trailing trucks with graduated lateral resistance.  I can’t find any examples where graduated lateral resistance was applied to two-wheel pilot trucks; fixed values in the 30 to 33⅓% appear to have been the norm.  Possibly graduation of lateral force was more difficult in the two-wheel case.  One may postulate that on tangent track, whereas the four-wheel truck was self-stable and so did not need significant lateral restraint, the two-wheel truck required a reasonable amount of initial lateral restraint to prevent it from being pushed to either side, as any lateral movement would generate a turning moment.  On the other hand, trailing two-wheel trucks would tend to be self-centring.

 

Regarding the “stable at 80 mile/h” design target, my understanding is that this arose from inertia considerations, allowing enough margin to ensure that there would be no problems at the envisaged 60 mile/h maximum operating speed.  Apparently, the UP had had some debate with Alco over this aspect during the design of the FEF 4-8-4, but had the mathematics to support its case.

 

Compared with the UP, the N&W appeared to work with higher axle loadings, lower factors of adhesion, and a tolerance for significantly higher lateral railhead forces.  (I understand that its 4-8-4 had around 50% lateral resistance for its pilot truck, probably well beyond what most roads would find acceptable.)  In the UP case, its approach to factor of adhesion was said to have been conditioned by representative Wyoming conditions, with the wind blowing the sand off the tracks as soon as it was laid.

 

 

Cheers,

[kgbw49]

I am just thinking the UP had so many fast freight engines that did welll with the 4-wheel lead truck that they just continued with it.

The 4-12-2 and 4-6-6-4 classes preceding the 4-8-8-4 class all performed satisfactorially as fast freiignt power.

4-6-6-4 units wer regularly assigned to passenger trains on the Los Angeles and Salt Lake as well as sections of The Portland Rose on the Oregon Railway & Navigation line.

Big Boy was designed to ostensibly operate smoothly at 80 mph, not that it ever attained that in operation.

But who knows how fast they might have run on fast freight on other sections of the railroad?

[daveklepper]

Overmod:  Terrific analysis.  And, despite the PRR, no other railroad can outrank bthe N&W for the very best engineering in stean locomotive design.  In my opinion.  The J, the A, and the Y each did their job with efficiency and low maintenance.

Maybe Ridley AT&SF came close.  But with double-heading just to avoid articulateds?

UP?  Why the four-wheel lead truck on the Big Boy?

The issues with PRR steam design and of the Associated railroads have been already examined, although the Birkshires (and the Erie's) and the 2-10-4 were great.

[Overmod]

II Phrogs

What of testing done on the Class A by other railroads  when tested (I believe it was the PRR, but feel free to correct me), where it was found quite wanting on grades in excess of 1%?

I cannot imagine that any Challenger equivalent of the class A would have any better grade performance, although there is a kinda-sorta higher adhesion on the rear couple of driver pairs due to the shorter overhang.  Very likely the 'difficulty' was adhesion on the forward engine, and a Challenger by definition would be worse there.

The A was among the first true high-speed simple articulateds, and with drivers at 70" was not intended for maximum tonnage on heavy grades -- that was a job for a compound Y with 56" drivers (and if you want adhesion, there are few articulated wheel arrangements capable of high speed that give more adhesion for a given locomotive weight than the 2-8-8-2 as used on the Y class).  Now, getting reliable 45+mph road speed out of a compound Y... which would have been absolutely everything a 50-freight-mph PRR could have wished for... is a different (and interesting) technical discussion.  But PRR, as you probably know if you were reading articulated-locomotive test results, had a very bad taste in its mouth after experience with the HC1, and famously preferred the C&O T to the N&W A even though the latter was 'all in the family'. 

[Challengers] have always struck me as incredibly well balanced locomotives, being relatively fast and powerful, but not at the expense of adhesion to the rails.[/quote]What they really represent is a compromise born of 1920s thinking: you "have" to have a four-wheel pin-guided lead truck for high speed, but you have to keep the absolute length of the locomotive low.  As soon as you know enough about proper two-wheel lead truck design and equalization, most of the real advantage of the Challenger leading end can be matched with one less driver pair, shorter piston rods, common rods and valve gear on both engines, etc.

At the back end there is no question.  A wide, deep firebox burns fuel more efficiently and transfers more of that heat to steam.  The only 'advantage' a shallow firebox with accordingly longer chamber (because TOF of the combustion plume is necessarily shorter) is if your fuel is largely friable and you don't care how much of it is wasted or blown out because it's so cheap and no one cares about pollution.

They were used by quite a number of railroads and as a result encountered a variety of running environments/conditions.

Most interestingly on railroads that used them effectively -- D&H perhaps being the poster child.

On top of all this, they did it all using some of the poorest quality coal out there, unlike some locomotives which required a more pampered diet of only "top shelf" coal.

That's inherent in rear stoking of the 'poor quality fuel' in question.  Much of the advantage of a deep firebox is lost if the fuel comes apart and reverses direction long before it can reach the grates and combust there.  Especially if the stoker is trying to distribute new fuel through a combustion plume full of what is essentially pulverized firing.

This was one of the things behind front-feeding stokers, a topic of considerable research in the late 1930s... and, as you might predict with more than a little unease, a colossal and apparently unmitigated failure in actual service.

It's a shame no one built a 70"-drivered double-Garratt.  THAT would have done it all on as poor a rank of coal or subbituminous as anything with a Mallet type chassis...

[Col Bob]

There is no "Best" although there is probably a "best for each railroad" given its topography, traffic, etc

For a comparison of Challengers see 4-6-6-4 "Challenger" Locomotives in the USA (steamlocomotive.com)

 

[Pneudyne]

BaltACD

 

 

 

 

Read in a book about the WM, the operating costs of their Challengers was such that the WM parked them several year BEFORE their equipment trusts expired - thus WM found it to their advantage to pay on the equipment trust without using the equipment that the trust was for.

 

 

That is not completely surprising.

 

It is likely that the Challengers were designed for, and primarily used for the road’s most important fast freight runs.  When built, they presumably provided better economics in that service than their steam predecessors.  But those fast freight runs were probably amongst the first to be dieselized, bringing the best immediate returns from the paradigm shift in motive power.  The less important remaining – and progressively reducing – runs available for steam locomotives were likely better served, with better economics, by smaller and older locomotives, typically of the two-cylinder type, than by underutilizing the Challengers, equipment trust costs notwithstanding.  That would be a reason for an early set aside as compared with older steam locomotives, even absent any operational or technical difficulties, or performance shortcomings, with the type.

 

Also, once a decision was made to cease maintenance work on steam locomotives, retirement dates may have been determined to some extent by the incidence of expiry of remaining flue time than by the relative utility or economics of any given type.  I suspect that the WM was not the only road to lay up steam locomotives until their equipment trusts were paid out – presumably they were not allowed to scrap them until then.

 

That the WM Challengers had operational and/or cost problems is not excluded and may well have been the case.  However, it was probably not a necessary precondition for the observed early layup of the fleet.

 

 

 

Cheers,

[Pneudyne]

Conductor_Carl

 

On a subject change, I feel like I have heard of the Western Maryland Challengers being a cut above. Something to the effect of them being higher horsepower than the others?

 

 

 

Western Maryland evidently made dynamometer car measurements on some of its steam locomotives, including the M-2 class Challenger, for which the drawbar power curve showed a peak of roundly 4700 hp at 50 mile/h, having reached 4250 hp at 30 mile/h.  The precision, repeatability and reproducibility of the curve and this number is unknown.  (Although back then, I doubt that latter-day ASTM-style precision and bias statements in respect of test methods and data were very common at all.)

 

The UP provided calculated curves.  For the “small” Challenger, these showed a drawbar power peak of about 4550 hp at 33 mile/h, declining to 4400 hp at 50 mile/h.  For the “big” Challenger, the numbers were 4750 hp at 34 mile/h, declining to 4450 hp at 50 mile/h.  How these calculated curves compared with the actuals is unknown.

 

I suspect that it would be unjustified to interpret these numbers with anything tighter than around a ±10% tolerance range.  In that light, we could say that all three were of broadly similar power output, with the UP “big” design being directionally more powerful than the “small”, and the WM design, closer overall to the UP “big”, having the peak of its power curve somewhat higher in the speed range than the other two.  Perhaps the last-mentioned gave rise to the “higher powered than the others” viewpoint?

 

Although it is the peak power numbers that create “bragging rights”, it probably requires some integration and weighting to derive the relative utilities of a set of power curves.  But to put that in perspective, in general, steam locomotive power curves were poorly matched to actual train haulage requirements -  i.e. they had much lower utility - as compared with those of modern traction.  That was shown quite clearly in P.W. Kiefer’s 1947 book “A Practical Evaluation of Motive Power”.  Perhaps one could say that attempting to rank the relative utility of steam locomotive power curves was itself a low-utility exercise.

 

 

Cheers,

[Pneudyne]

Moving from a component analysis to an overall viewpoint, the locomotive needs to be able to do the intended haulage job with acceptable reliability and maintenance costs, burn the available/preferred fuel, be an acceptable vehicle on the track throughout its speed range, and to the extent reasonable, follow the particular railroad’s established patterns and preferences.

 

In particular, the firebox/boiler and running gear must fit each other.  That requirement could entail that one or both depart, a little or perhaps a lot, from particularist ideals as to how they should look.  Detail design then becomes a series of tradesoff.  In some situations, it is likely that more than one design approach would do the required job at acceptable cost.

 

In the case of the x-6-6-y articulated, there was a tradeoff to be made between the pilot truck and firebox configurations.  If a four-wheel pilot truck was desired to keep lateral railhead forces within the designated system boundary, then a long, shallow firebox was dictated.  If a deep firebox was desired for the best combustion of good bituminous coal, then a two-wheel pilot truck was dictated.  Neither tradeoff forced a “fatal” component choice.  Also, it is not clear that the deep firebox was necessarily better when it came to burning lower quality sub-bituminous coal, or oil.  With the latter, firebox length appears to be a significant parameter, in which case the Challenger firebox might have been well-matched to the task.

 

Duffy, in his paper “Technomorphology and the Stephenson Traction System” (1), made some interesting observations on the components and systems viewpoint, as follows.

 

‘Hence, a transformed component must be related to the larger system if its significance is to be gauged, because assessment of a particular technology can change if the view embraces the whole system instead of being limited to the component. The 'scale' of the view taken—whether 'components scale' or 'systems scale'—decided policy towards steam traction in the 1940s and 1950s.

 

‘Those engineers who took the 'systems' view (which embraced the whole railway with its role in the economy and general technology) recognised the impending obsolescence of steam power in time to introduce superior traction modes in an economical manner. Those who failed to take the broad view, such as Chapelon and Riddles, wasted valuable resources and time in developing steam locomotives when they were obsolete. There were also shifts in the scale of the view taken within the steam traction machine-ensemble. Engineers such as Chapelon and Lawford Fry, tended to design high-powered, high thermal efficiency, often complex types. The more modern minded, who took the larger scale view embracing the entire machine-ensemble, simplified the locomotive and reformed the system through better management.

 

‘Some belonged to the Churchward, Gresley, Bulleid tradition, believing that it would be through ever-increasing ingenuity in design that the locomotive would rise to increasing heights of performance. Others were inclined to the Stanier, Thompson, Riddles opinion that it would be through simplification in design and good organisation for maintenance that progress would be made.’

 

Against that, it seems reasonable that locomotive design details such as firebox shape may have had less significance in the whole system, that is with the locomotive as a part in the whole railroad system, than when looked at individually.

 

Back in the 1990s, there was a magazine article (Trains or RF&RR – I don’t recall which) on UP 3985 either by Steve Lee (of the then UP steam team) or including commentary from him.  There was some comparison with the N&W 2-6-6-4; in respect of the firebox shape, Steve Lee’s viewpoint was that in practice, it didn’t matter.  He might not have been without bias, but on the other hand, in big picture system terms, he was probably right.

 

In respect of the UP, I understand that Jabelman would have acquired diesels has they been available rather than the additional Challengers, Big Boys and FEFs in 1944.  Although he was a steam locomotive designer, he evidently took the broad view that modern traction was a much bigger step forward in terms of the whole railroad system than would be any practicable and significant improvement in the steam locomotive.

 

Regarding two-wheel versus four-wheel pilot trucks, it is evident that to a first approximation, for the same lateral guiding force the latter would exert half the lateral railhead force.  (Accepting that variations in moment arm lengths, etc., would introduce some variation away from the 1:2 ratio.)  As the lateral railhead forces increase with the square of the speed, the four-wheel truck would allow 1.4 times the speed of a two-wheel truck for the same lateral railhead force.

 

One comparative datapoint comes from the (rather pointless) British Railways standard steam locomotive programme.  There it was determined that any locomotives that operated habitually above 60 mile/h would be fitted with four-wheel pilot trucks.

 

Some late four-wheel pilot trucks had variable lateral resistance.  I don’t know the numbers for the UP “big” Challengers, but they were probably similar to those for the Big Boy, namely 18% initially, increasing to 33%.  Alco seemed to use this approach in conjunction with its lever principle in which all but the trailing set of drivers had lateral motion devices.  I don’t know if progressive lateral control was also applied to two-wheel pilot trucks, but if so, I have not seen mention of it.  I think that the N&W A had a fixed 30%, but that needs to be confirmed.  A possible factor is that unlike a four-wheel truck, a two-wheel pilot truck is not self-stable.  Being pushed, as it were, it would be inclined to swing one way or the other, so that the lateral controls would need to be strong enough to keep it steady on tangent track.

 

As said, turning an articulated locomotive front unit into a curve requires less lateral force than turning a rigid locomotive, but the front unit has the job of acting as a pilot for the remainder of the locomotive.  Also, the front units were generally viewed as being potentially problematical in respect of lateral stability, so adequate pilot truck guiding force on tangent track was required.

 

The European approach of having a two-wheel pilot truck linked to the leading driving axle in order to provide the latter with some lateral motion probably offered some improvement, but it would seem that the use of a lateral motion device on the leading driving axle was a better solution, in that the latter could then adjust itself independently, allowing optimum distribution of lateral railhead forces.

 

I was not aware that trailing truck stability was a major issue, except maybe for the Lima articulated type as used on the A-1 & co.  It would appear that it needed enough lateral resistance to keep it stable on tangent track, but not so much that it undid the good work done by the pilot truck in curving.  That seemed to mean that it had about half the lateral resistance of the pilot truck.  The Big Boy had 10% initial, increasing to 18%.  (Trailing trucks competing with pilot trucks was a problem with bidirectional rigid-frame electric locomotives, such as the 1-D-1 and 2-D-2 types, but that is a separate issue.)

 

An interesting aspect of the N&W A was that it had a single-plane articulated joint.  This, when it had relatively limited clearances that severely restricted pitch axis motion, was held to offer a significant improvement in front unit stability.  N&W appears to have been the first to use it, although it did not call it out as a significant feature, nor, as best I can ascertain, did it patent it.  Alco had addressed the problem differently in the case of the original UP Challenger.  The conventional two-plane articulation joint was friction damped on its pitch axis.  This though caused undue stress on the built-up front unit frame.  Alco first used the single-plane joint on the D&H Challenger in 1940.  Lima used it on the C&O Allegheny; Baldwin’s first use was on the B&O 2-8-8-4.  A perhaps surprising application was on the final Baldwin 2-6-6-2 batch for the C&O.  Way back when (late 1980s, I think), I was looking at the example in the Baltimore museum, and did a double take when I saw the articulation joint.  And this was with built-up, not cast frames.

 

 

(1)   M.C, Duffy (1982) Technomorphology and the Stephenson Traction System, Transactions of the Newcomen Society.

 

 

Cheers,

[Overmod]

Keep in mind that the effective tube/flue length in any modern boiler is in the range of 20 - 24'.  Anything longer than that is a waste, even if used in a Chapelon or Porta sectional arrangement.

The issue with burning subbituminous 'correctly' is that the fuel is extremely friable, and breaks up and levitates in the combustion plume.  You will see ignorant railfans saying something like "40% of the coal never hits the grates" and indeed it doesn't, but it burns similar to carbureted oil, with some of the same luminous-flame advantage that oil has.

In order to get this effect with a typical 'deep' firebox, you would need a method of firing either capable of placing fuel far down under the arch, or feeding at the throat as many methods of oil firing do to get the longest plume.  In practice that would be more difficult than the methods used on Challengers.

There are a couple of presumptions: that the lavish amount of fuel to produce equivalent mass flow of steam is available, and that the net cost of all the fuel is lower than bituminous or oil.  A more important consideration (since 1970, nearly a decade after it ceased to matter on UP) is the extraordinarily dirty exhaust from that method of firing using nothing more complicated than a regular stoker arrangement for control.  Careful tinkering with the secondary-air arrangements and providing effective combustion-air preheat might go a long way toward addressing this stuff.

You wouldn't fire an A on subbituminous or any other low-rank coal; it wasn't designed for that.  It might be said that the Allegheny could be run on lower-quality coal -- but I suspect it, too, would not do well.  That might be different if you arranged to feed the fuel from the throat, something that was the subject of a great deal of development in the late Thirties (and that, as discussed in a couple of other threads, might have been one of the great epic failures of steam technology in practice...)

Of course, there is a better answer: the Garratt.  That gives you unhindered access to a grate that can be as low and wide as your loading gage permits, combined with a barrel diameter uncompromised by driver or frame height and adequate room for multipass Franco-Crosti economization, effective air preheat, etc.  A couple of 2-6-6-2 engines and you have everything a large engine could want for...

[II Phrogs]

Forgive my ignorance but would a deep firebox as described be just as useful when using the lesser quality coal utilized by many of the roads that employed Challengers? I was under the impression that the shallow firebox was necessary to increase volume to make up for the lower-energy coal being used, and as such were a necessary compromise.

For the sake of discussion how would a N&W A run using UP Sub-Bituminous coal, or in an even more extreme case Northern Pacific Rosebud coal? Conversely, how would a wide, shallow firebox Challenger run using the high BTU coal used on the N&W?

[Overmod]

There are a number of references, none yet quite rigorous, which point out the superiority of a proper deep firebox with circulators over something with a wide grate 'over the drivers'.  This is further enhanced if active circulation is provided in the water legs (a la Cunningham circulator, which draws from a downcoming region in the convection section of the boiler, and uses a jet pump to distribute it through nozzles in the outer wrapper above the mud ring).

The deep firebox implies greater mass on the trailing truck, both from the additional metal structure and weight of water.  Note that on C&O, which had dramatically high axle-load capacity, the Alleghenies have a six-wheel trailing truck.

Rear-end stability on a deep-firebox engine of suitable capacity is almost incomparably better than a Challenger.  Look at the Bissel formula that keeps the truck wheelbase 'normal to the railhead' in curves, then extend the truck out so rear bearing and steering forces are as far outboard and to the rear of the chassis as possible, and angle the restoring-force devices (usually rockers or segments of gears) to match the swing radius at the rear.

In a pinch, you could use the dodge that was introduced in the 'intermediate' Berkshire trailing truck frame, when a long frame pivoted at the original 'articulation' point was used as a Delta-style trailer.  This was treated dynamically as a long 2-wheel Delta trailer, with the leading axle only weight-bearing -- it could float laterally on a pair of hardened-steel rollers independently of truck-frame angularity.  Any of the subsequent schemes of lateral-motion compliance could be used on such an axle if desired.

The only American engines with a deep firebox and divided drive that used a four-wheel lead truck were the PRR Qs, and those did not have the 'compound-pendulum' guiding concerns of a Mallet-style chassis.  All the six- and eight-coupled simple articulateds with deep fireboxes used some form of two-wheel Bissel lead truck, and in at least one situation (the B&O EM-1s in M&E service) this caused a service-speed reduction -- I do not know whether this was actually based in engineering analysis or just advisory/CTA.  The primary reason for the two-wheel truck is simple: it reduces the overall length of the engine at the front, rather than by siamesing the firebox over the drivers at the rear; a secondary reason is that all four mains can be the same dimensions for balance without needing to resort to an extended piston rod and crosshead guides a la PRR T1.

We need not look further than the N&W A (and the late Y-class) for a 'correct' two-wheel high-speed lead truck.  It could be argued that LS&MS had some careful design on their 2-6-2s (which were supposed to be among the world's fastest engines when introduced), and that New York Central kludging them into rather inferior Pacifics after the disaster with Wilgus' "1-D-1"s having to have four-wheel engine trucks shoehorned under them was 'unnecessary surgery' from an actual dynamic performance point of view -- but I do not know for sure either way.

Certainly the A was good for 70mph as designed, and I strongly suspect the last 5 (with the lightweight rods) had some 'breathing' done on their lead trucks for better curve entry and mitigation of leading-driver-pair flange forces (the mass of the chassis being considerably less than that of a conventional locomotive with the same truck and driver spacing).

You run into the same overweight problem with any real high-speed articulated just as Lima did with the Allegheny/Blue Ridge types: the weight is in the high-capacity streamlined steam and exhaust piping arrangement, and this would apply to a proper Challenger.  We note that for high-horsepower high-speed designs, nothing more than six-coupled engines are used -- there is no particular reason why you couldn't make a ginormous 2-8-8-x with 76" drivers except that it would be too long, relatively heavy, and possess a diabetic water rate requiring auxiliary tenders to get even mediocre range between water fills.  Any single-unit steam locomotive, reciprocating or turbine, hits that limit somewhere around 8000dbhp, and of course many hit it before.  This, for example, is why we all laughed so hard at the railfan version of Chapelon's Big Boy 'improvement regimen' that would have it producing "10000hp" or greater -- even with proper Chapelon compounding and American detail design for high thrust, you'd have no fun running it at that power.

For fun, start with the rough dimensions of a N&W A (or Seaboard R-2) and give it a double-Belpaire chamber.  This would give you up to 76" potential driver diameter to play with, but I wouldn't go above 72" or so with modern dynamic balancing and thin-section rods.  Use the Allegheny piping dimensions, with some reasonable boiler pressure that doesn't cause ridiculous maintenance requirements (this would likely still be somewhat south of 300psi, and certainly no higher!) and carefully design your chests and valve size to optimize high-speed flow.  (You will definitely want reversible compression control on the forward engine, and that indicates you'd likely want it on all four cylinders).  With proper Snyder combustion-air preheating and Cunningham circulation, this might keep the required firebox+chamber dimensions in line with what a four-wheel truck could bear, and the corresponding grate area might make the engine more flexible to fire at less than full load. 

[II Phrogs]

Pneudyne

 

Maybe look at the 4-6-6-4 vs. 2-6-6-4 comparison this way:

 

Start with the UP “big” Challenger of 1942 as a baseline.  Then design a 2-6-6-4 (or 2-6-6-6 if needs be) within the following constraints:

 

1.     The same driving axle load (67 500 lb)

2.     Lateral railhead forces (during curving and arising from restraint of yaw oscillation) no higher at any speed, recalculating the 4-6-6-4 for any lateral control improvements developed for the 2-6-6-4 case that could also be applied to the 4-6-6-4.

3.     No perceptible difference in whole locomotive and front engine unit stability at any speed,

4.     The same factor of adhesion.

 

What then would be the likely advantages conferred by the 2-6-6-4 (or 2-6-6-6), realizable in daily service?

 

(I don’t know the answer, by the way.)

 

 

 

Cheers,

 

I thought this was an interesting set of questions and wanted to revisit it to see if anyone with more knowledge on the subject has any answers? I feel that discussing and finding answers to specific questions such as these challenges (hehe) people to push beyond their natural biases and overcome the usual "well this locomotive is clearly best since it ran in the same geographical location that I happen to be from"! 

[Conductor_Carl]

Bringing this thread back a little bit, does anyone have the unit costs of these? I've seen some numbers thrown around for NP Z-6's (185,000 ish) the UP CSA-1's (130,000 ish) and the last UP Challengers (225,000 ish) but apart from the Z-6 I dont really have faith in these numbers. Would be interesting to see how costs stack up. 

[Pneudyne]

 

Kratville, in his book on the UP Challengers, said the following in respect of the six “diverted” to the DRGW:

 

‘The Union Pacific applied in late 1942 for the 3975-3999 series, the engines being finally delivered in mid-1943.  The road desired more units but that is the most the W.P.B. calculated could be put through Alco in the light orders for other roads.

 

‘At the same time, the W.P.B. was assisting the Denver and Rio Grande Western gear up for the eventual Pacific Theatre operations transportation shift and added six additional Challengers to the U.P. order.  The D.&R.G.W. made it clear that it probably would not want the six units after the war particularly because of having to keep parts in stock for just six locomotives.  The Union Pacific did not offer to take them either since by this time Jabelmann was already laying out his own plans to dieselize the road right after the war.’

 

Thus, it looks as if it was predetermined that the DRGW would not keep these locomotives beyond the end of WWII.

 

 

Cheers,

 

 

 

 

 

Returning to the DRGW six Alco Challengers, ‘Train Shed Cyclopedia (TSC) #45’ has a tabulation of steam locomotive orders and deliveries from 1939 onwards.  It shows that DRGW placed an order for six 4-6-6-4s from Baldwin in 1942 May, but that this was not approved by the WPB.  An alternative order for six Alco 4-6-6-4s was shown as placed in 1942 June, and approved by the WPB.  The UP order for 25 was shown as placed in 1942 February and approved by the WPB.

 

The UP “big” Challenger was a substantially new design, but it had slipped in before the WPB constraints were applied.  TSC #45 shows that the initial order for 20 was placed in 1941 May for 1942 delivery.

 

If one wanted to make a broad classification of the Challengers by basic design and builder, then the following, with arbitrary numbering, might work:

 

Alco type 1A                 1936                 UP, WP

Alco type 1B                 1940                 D&H, Clinchfield

Alco type 2A                 1936                 NP, SPS (earlier)

Alco type 2B                 1941                 NP, SPS (later)

Alco type 3                   1942                 UP, DRGW

 

Baldwin type 1              1938                 DRGW

Baldwin type 2              1940                 WM

 

 

Some of the Challengers were described in ‘Railway Age’ and ‘Railway Mechanical Engineer’ articles at the time of their appearance.  Known such articles are:

 

UP 'Small'	RA	1936   December 19	pp.900-903
UP 'Small'	RME	1937   January	pp.1-7
NP	RA	1937   March 06	pp.389-391
NP	RME	1937   April	pp.160-163
DRGW	RA	1938   July 09	pp.42-44,70
DRGW	RME	1938   September	pp.323-329
D&H	RA	1940   August 10	pp.207-218
D&H	RME	1940   September	pp.337-344
WM	RA	1941   January 25	pp.209-215
WM	RME	1941   February	pp.45-52
UP 'Big'	RA	1942   October 03	pp.516-519
UP 'Big'	RME	1942   October	pp.413-417

 

As I think is well-known, both journals are available at the Internet Archive, RME under its later name of ‘Railway Locomotives and Cars’.

 

https://archive.org/details/pub_railway-age?sort=-date&and%5B%5D=year%3A%221937%22

 

https://archive.org/details/pub_railway-locomotives-and-cars

 

 

 

Cheers,

[Pneudyne]

II Phrogs

How did the relatively poor quality coal used by the Northern Pacific impact their performance, and by extension did the conversion to oil for the SP&S locomotives give any advantage or boost to their performance? 

 

 

Nominally at least, one might reasonably expect that the much larger firebox volume and grate area of the NP 4-6-6-4, as well as other changes such as in the type of grate used, as compared with the UP original, was intended to compensate for the poorer quality coal used by the NP.  Whether in practice it  under- or overcompensated is unknown.  Possibly it did both, at different points in the power vs speed curve.

 

With oil firing, grate area is a meaningless parameter.  Firebox volume and firebox length appear to be more important.  Nonetheless, most oil fired locomotive fireboxes seem to have been designed with possible conversion to coal firing in mind, so that a notional grate area was typically quoted anyway.

 

Many of the UP Challengers were converted to oil firing.  None of the available information suggests that there was any differentiation between the oil-fired and coal-fired versions in terms of load schedules or expected end-to-end speed performance.  A reasonable inference is that in oil-fired form, the firebox and draughting details were chosen generally to match the coal-fired form in overall performance, notwithstanding the higher heating value of the fuel oil, say around 18 000 BTU/lb as compared with the 11 800 BTU/lb of the coal used by the UP.

 

Possibly the SPS oil-burning Challenger were setup to more-or-less match those on the NP.  The NP rosebud coal was said to have a hearing value in the range 6200 to 8000 BTU/lb ex mine, 10 000 BTU/lb when dried.  (Was it dried before use?)

 

A different view was expressed by LeMassena in his book ‘Articulated Steam Locomotives of North America’.  Therein he said: ‘It is little recognized that these SP&S articulateds were among the most powerful steam locomotives ever constructed, and they were able to deliver more power than those of the NP because the latter burned a low grade of coal instead of oil.’  One supposes that LeMassena had quantitative evidence to support that statement, and did not simply infer it from the relative fuel heating values.

 

An interesting case was the WP 2-8-8-2, built as an oil-burner, but said to have a notional grate area of 145 ft².  The firebox was 204⅛ inches long x 102¼ inches wide.  The same design was used as the basis for the DM&IR 2-8-8-4, which burned coal with a heating value of 13 500 BTU/lb.  Here the firebox was slightly longer, at 210⅛ inches, but the same width.  But the grate area, presumably chosen to suit the coal used, but also to allow the use of a Gaines wall, was 125 ft²; it did not occupy the whole firebox length.

 

 

 

Cheers,

[Pneudyne]

 

Kratville, in his book on the UP Challengers, said the following in respect of the six 'diverted' to the DRGW:

 

‘The Union Pacific applied in late 1942 for the 3975-3999 series, the engines being finally delivered in mid-1943.  The road desired more units but that is the most the W.P.B. calculated could be put through Alco in the light orders for other roads.

 

‘At the same time, the W.P.B. was assisting the Denver and Rio Grande Western gear up for the eventual Pacific Theatre operations transportation shift and added six additional Challengers to the U.P. order.  The D.&R.G.W. made it clear that it probably would not want the six units after the war particularly because of having to keep parts in stock for just six locomotives.  The Union Pacific did not offer to take them either since by this time Jabelmann was already laying out his own plans to dieselize the road right after the war.’

 

Thus, it looks as if it were predetermined that the DRGW would not keep these locomotives beyond the end of WWII.

 

 

Cheers,

[drgwcs]

II Phrogs

 

 

Pneudyne

 

 

 

 

Just wanted to say thank you, this was both a highly informative and entertaining read and gives a good understanding of. the differencies and similarities between the various Challenger designs. 

If I am remembering correctly, I believe the D&RGW used a handful of the later Alco designed Challengers which were diverted from a UP order to make up for a shortage in motive power on the D&RGW during the war. To your knowledge, how did these UP Challengers fare in service on a different railroad? From what I've read the D&RGW wasn't particularly enamored with these locomotives. Was this due to any inherent flaw in their design, or was it more that the Challengers were delivered in lieu of the FT diesels that many roads were trying to get their hands on around this time? Is there any data or testimony on how these locomotives stacked up to the Baldwin 4-6-6-4's already in use on the D&RGW?

 

Rio Grande had originally wanted to order some additional Baldwin 4-6-6-4's. The war production board would not do so and diverted some from a UP order. The Alco UP design challengers diverted to the Rio Grande were on lease from the War Production Board. The Rio Grande crews did not like them. From what I remember reading (but I can't remember the source and was trying to find it) the crews found the pullling power lacking along with the ride qualities compared to the Baldwins. I am trying to remember but it seems like they may have had some issues with the centipede tenders. UP and Rio Grande approaced their usage of challengers a bit differently. UP tended to run higher speeds- Rio Grande slower and longer trains. After the war the Alcos were given back to the War Production board and would up beind sold to the Clinchfield.

[Leo_Ames]

I have no insight on how they were viewed by the line, but Trains reported at the time that the Rio Grande returned them to the War Assets Administration since they were surplus to their needs with the postwar traffic decline and they didn't want to purchase them.

[II Phrogs]

Pneudyne

 

Just wanted to say thank you, this was both a highly informative and entertaining read and gives a good understanding of. the differencies and similarities between the various Challenger designs. 

If I am remembering correctly, I believe the D&RGW used a handful of the later Alco designed Challengers which were diverted from a UP order to make up for a shortage in motive power on the D&RGW during the war. To your knowledge, how did these UP Challengers fare in service on a different railroad? From what I've read the D&RGW wasn't particularly enamored with these locomotives. Was this due to any inherent flaw in their design, or was it more that the Challengers were delivered in lieu of the FT diesels that many roads were trying to get their hands on around this time? Is there any data or testimony on how these locomotives stacked up to the Baldwin 4-6-6-4's already in use on the D&RGW?

[Pneudyne]

 

Although there is likely insufficient operational and performance data to undertake a full comparison of the several Challenger types, at least a relatively simple dimensional comparison may be made.

 

The original UP Challengers of 1936 were built to a 65 000 lb axle loading limit, and had 386 000 lb on drivers.  The grate area was 108 ft² (a number inherited from the 4-12-2).  Boiler pressure was 255 lbf/in², cylinder dimensions were 22” x 32”, with 69 inch drivers.  Alco and UP may have been concerned about front engine stability at higher operating speeds.  So, although a conventional (two-plane) articulation joint was used, it was friction damped on its pitch axis.  The pilot truck also had pitch damping.  The choice of four-wheel pilot truck was part of this, although the total weight, coupled with the relatively modest weight on drivers imposed by the axle loading limit probably made the use of the four-wheel truck mandatory anyway.  Apparently, reduction of rail thrusts was one of UP’s objectives, and it was anyway accustomed to using four-wheel pilot trucks on its fast freight locomotives.  To put the UP Challenger adhesive weight into perspective, the N&W 2-6-6-4, as first built in 1936, had 430 000 lb on drivers, suggesting that it was built to an axle loading target or limit of 72 000 lb, more than 10% higher than the UP number.

 

The following NP Challenger was a derivative of the UP design, with a much larger (152 ft²) grate area to facilitate the burning of rosebud coal.  Thus, it was heavier, with an adhesive weight of 435 000 lb, suggesting a target axle loading of 72 500lb.  Perhaps to take advantage of this, the cylinder diameter went up to 23”.  Boiler pressure was 250 lbf/in².  It retained the same pitch damping arrangement as on the UP locomotive.  The longer firebox meant a longer engine wheelbase, 61’10” as compared with 59’11”.

 

The 1938 WP Challenger appeared to have been more-or-less the UP design with some detail changes, and somewhat heavier.  Adhesive weight was 416 000 lb.  Perhaps the axle loading target was 69 500 lb or thereabouts.  Cylinders were the “standard” 22” x 30”, with 70 inch drivers and a 265 lbf/in² boiler pressure.  Whether it had the pitch damping arrangement is unknown, but as a clone of the UP original it does seem likely.

 

The 1940 D&H Challenger was a modernization of the original UP design, retaining the same basic dimensions, including the 108 ft² grate area.  The driving axle load target was 68 000 lb, and the adhesive weight was 406 500 lb.  That was distributed asymmetrically, 201 000 lb on the rear and 205 500 lb on the front, this to provide nominally equal distribution on a 1.5% upgrade.  Boiler pressure was 285 lbf/in², and cylinders were 20½” x 32”.  These dimensions appear to reflect the D&H leaning towards higher-than-typical boiler pressures.  Mechanically, it had cast frames, something that the earlier Alco Challengers did not, but probably should have had.  Also, it had a single-plane articulation joint, which dispensed with the need for pitch axis damping.  This type of joint seems to have originated with the N&W 2-6-6-4 in 1936.  It might have been a GSC idea, something that took advantage of the extra strength of cast frames.

 

The second NP Challenger design in 1941 was a little longer than its original, with a total engine wheelbase of 62’2”.  The boiler had the same grate area as before, but was slightly longer.  Adhesive weight went up to 444 000 lb, suggesting an upward axle loading creep to 74 000 lb.  Cylinders were still 23” x 32”, but drivers were 70 inches (with slightly longer driving wheelbase) and boiler pressure was 260 lbf/in².  As far as I know, NP retained the two-plane articulation joint.  Whether it also retained the pitch axis damping is unknown

 

The 1942 Clinchfield Challenger was the D&H design with detail changes.  A 70 000 lb driving axle load allowed a 420 000 lb adhesive weight.  It reverted to the “standard” 22’ x 32” cylinder size with 265 lbf/in² boiler pressure.

 

But the 1942 UP “Big” Challenger was virtually a complete departure.  Rather it seemed to have been more-or-less a three-quarter sized edition of the Big Boy.  Grate area was 132 ft², boiler pressure was 280 lbf/in², cylinders were 21” x 32”, and drivers were still 69”.  The axle loading target was 67 500 lb (a number which UP appeared to have adopted with the original FEF), and adhesive weight was 404 000 lb.  They had cast frames and single-plane articulation joints.  Also they had the full Alco-Blunt system of lateral controls, as did the Big Boy, and which UP had adopted with the FEF-2 in 1939.

 

Baldwin’s first of two Challenger designs was in 1938 for the DRGW.  It had a 136.5 ft² grate area, 255 lbf/in² boiler pressure, 23” x 32” cylinders, 70” drivers and 438 000 lb adhesive weight, suggesting a 73 000 lb axle loading target.  It does appear that Baldwin was aiming at a somewhat larger locomotive overall than the baseline Alco design.  It did have cast frames, but with a conventional two-plane articulation joint.

 

Baldwin’s second Challenger design for WM in 1941 was somewhat smaller, broadly similar in size to the Alco baseline design.  Grate area was 118.8 ft², boiler pressure was 250 lbf/in², cylinders were 22” x 32”, and drivers were 69”.  Adhesive weight was 402 000 lb, suggesting an axle loading target of 67 000 lb.  It had cast frames with a conventional articulation joint.  It did have a different steam distribution system to the DRGW design, though.

 

Notwithstanding the variations indicated in the foregoing, all of the Challengers had one common dimension, namely 102 inches boiler maximum outside diameter (BMOD).

 

Note also that later batch builds of a given design sometimes had variations, particularly in respect of weight.

 

I should be wary of drawing too much from the above very simple comparison, which does not at all address the thermal aspects.  Nonetheless, on the basis of adhesive weights alone, it is not difficult to see a possible reason as to why DRGW preferred its own design over the UP “Big” design foisted upon it by the WPB.  Simply its own design would have been able to reliably start and accelerate a heavier load on the ruling grade.

 

UP does seem to have been more concerned about rail stress than most.  I suspect that the 67 500 lb axle loading that it allowed for its late steam designs, conservative relative to what other large roads were doing, did not reflect a lighter track and roadbed, but a concern as to what happened at higher sustained speeds, given that rail stresses increase as the square of the speed.  Its adoption of the Alco-Blunt lateral controls is another indicator.  Possibly the severe winter conditions to which most of its main line was exposed was another factor in its conservatism.

 

The original Alco UP Challenger was somewhat “shaped” by the fact that at least to a first approximation, it was an articulated derivative of the UP 4-12-2.  The latter was built to a nominal axle loading of 59 000 lb, although the trailing truck was over at 60 000 lb.  Thus, any likely derivative was almost certain to have an additional carrying axle.  The four-wheel pilot truck came into UP freight locomotive practice as a matter of necessity, with the 4-10-2.  This was intended to be as much like its 2-10-2 as possible, but the extra weight, and probably the extra overhang, of the three-cylinder assembly, necessitated a four-wheel pilot truck.  This was carried over to the 4-12-2, and apparently the UP found it advantageous in general.

 

 

 

Cheers,

[Conductor_Carl]

Well, that ain't good

[BaltACD]

Conductor_Carl

...

On a subject change, I feel like I have heard of the Western Maryland Challengers being a cut above. Something to the effect of them being higher horsepower than the others?

Read in a book about the WM, the operating costs of their Challengers was such that the WM parked them several year BEFORE their equipment trusts expired - thus WM found it to their advantage to pay on the equipment trust without using the equipment that the trust was for.

[Conductor_Carl]

Comparing the different Challengers may be a bit hard as they are geographically far flung enough that their usage may vary, and what works for the Northern Pacific may not be applicable to the Delaware and Hudson.

I think that there are two ways that you could cut this to be interesting. 

1. Look at the usage of the UP challengers on the Rio Grande and Clinchfield. If they worked essentially just as well on all three roads then that is a lot of geographical distance and perhaps different usage covered by one loco type and would speak to a universal quality.

2. Compare the UP Challengers on the Rio Grande to the Rio Grande Challengers. This compares two the two different Challenger  different builders (ALCO and Baldwin) on essentially the same road and conditions. 

On a subject change, I feel like I have heard of the Western Maryland Challengers being a cut above. Something to the effect of them being higher horsepower than the others?