Comparing The Challengers

[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?

 

[kgbw49]

Great Northern also owned two Z-6 oil-fired Challengers that were bought from subsidiary Spokane, Portland & Seattle that were numbered 4000 and 4001.

Eventually as it dieselized the Great Northern sold them back to SP&S.

But Great Northern is not typically though of as a Challenger owner but it was in the 4-6-6-4 club!

https://www.deviantart.com/avalanch11/art/Great-Northern-4-6-6-4-4000-Steam-Locomotive-996878578

 

[II Phrogs]

BigJim

I would suggest that you try to find and read a copy of Eugene L. Huddleston's book "The World's Greatest Steam locomotives". In this book Mr. Huddleston compares the "Allegheny", the Class A, and the Challenger. Plus, some other tid-bits!

 

Many thanks for the reading suggestion, I am ever on the prowl for a new book to expand my collection/knowledge and will certainly look into acquiring a copy of this one! Additonally thank you and others for the responses regarding the Class A 2-6-6-4's, I do hope I did not come off as disparaging these fantastic locomotives, I wish only to expand my understanding of these fascinating machines. 

If I may turn back to my initial question and request your and others knowledge on the subject, I have one standing question in regards to the Z-8 Challengers of the NP and SP&S. As I understand it, the SP&S challengers were converted to burn oil, however kept the gargantuan fireboxes of the original NP design. Would this conversion to oil have any appreciable impact on the performance of the locomotives? What differences might one expect going from burning the incredibly low quality coal used by the NP to burning oil? Any insight into this is greatly appreciated.

[BigJim]

II Phrogs

May I ask what advantages those might be? The only inherent disadvantage of the 4-6-6-4 when compared to the 2-6-6-4 that comes to mind (though I am not in any way an expert on the subject, hence my curiosity) is the shallow firebox of the Challengers, which extends over the rear drivers. While, at least in the case of the N&W A Class, the 2-6-6-4 seems to produce a fair bit more tractive effort, it also seems to have a surprisingly low adhesive factor, would that not balance things out in the end?

I would suggest that you try to find and read a copy of Eugene L. Huddleston's book "The World's Greatest Steam locomotives". In this book Mr. Huddleston compares the "Allegheny", the Class A, and the Challenger. Plus, some other tid-bits!

[BaltACD]

Erik_Mag

Diesel locomotives can produce close to rated horsepower at any speed from the speed at which maximum continuous tractive effort to somewhere near maximum rated speed. Furthermore, by simply changing the gear ratio, a given locomotive mode can be set up for drag service or high speed service. Finally, diesel locomotives can be M.U.'ed so it is simple to lash up just the required number of units to haul a train. Fuel is pretty much the same for all RR's.

Steam locomotives are much less flexible, requiring different designs for flat land running versus mountain hauling, hauling long trains versus short trains, etc. Fuel could be high quality bituminous, lignite or bunker C, each requiring different firebox designs.

To be fair, lack of standardization in steam was driven by hard requirements as well as the whims of the various mechanical departments.

One overlooked area of the differences between steam and diesel.  Steam engines, besides containing the air compressors and the brake valve - did not factor into the braking equation for the train it was hauling.  Using he Independent brake would bring the brake shoes in contact with tires on the drivers.  The tires had been heated to create a shrink fit with the driver wheel when the tire cooled.  If too much Independent brake was used the tire(s) could expand from the heat and leave the driving wheel.

Diesel have the use of Dynamic Brakes to retard the movement of the train and thus the locomotives become a bigger factor in the braking of trains, especially in mountainous territory..

In steam days the Retainer Valves had to be manipulated by the crew to 'reatin' brake applications for a period of time while the trainline was being recharged after a brake application was released.  Setting the Retainers up or down was a manual fuction performed by the brakemen on a train.

[Erik_Mag]

Diesel locomotives can produce close to rated horsepower at any speed from the speed at which maximum continuous tractive effort to somewhere near maximum rated speed. Furthermore, by simply changing the gear ratio, a given locomotive mode can be set up for drag service or high speed service. Finally, diesel locomotives can be M.U.'ed so it is simple to lash up just the required number of units to haul a train. Fuel is pretty much the same for all RR's.

Steam locomotives are much less flexible, requiring different designs for flat land running versus mountain hauling, hauling long trains versus short trains, etc. Fuel could be high quality bituminous, lignite or bunker C, each requiring different firebox designs.

To be fair, lack of standardization in steam was driven by hard requirements as well as the whims of the various mechanical departments.

[CSSHEGEWISCH]

More proof of the lack of standardization in steam.

[Flintlock76]

timz

How wanting? PRR tested the A, but none of us has any idea what tonnage it pulled on what PRR grades, or failed to pull. PRR decided not to bite, but none of us knows why.

I've said this before and I'll say it again.  I have no way of proving this but I suspect the PRR (with Baldwin looking over their shoulder) after testing an N&W Class A ( and Class J for that matter) didn't want to admit those "hillbillies" down in Roanoke were better at steam locomotive design than they were!    

Corporate ego may not have played a role here but I wouldn't discount it entirely.  The PRR didn't call itself "The Standard Railroad Of The World" for nothing!  

Just a "might have been" to think about.  Post-WW2 and flush with money and not having bought any new steam locomotives since 1930 the Jersey Central was considering buying Challengers for their coal drags but not for long.  In the end they bought F3 diesels. 

[kgbw49]

Here is a picture of Challenger 3935 on Train 717 The Los Angeles Challenger in 1940.

https://www.railpictures.net/photo/850086/

[BigJim]

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%?

No need for other RR's. Just listen to any of the actual recordings of the Class A's on the Blue Ridge grade, which by the way is in excess of 1%, and you will find that they are quite surefooted! Then, go find out what kind of tonnage they hauled on the way out of Williamson, WVa or Crewe, Va. I am sure that you will find that you need to do tons more researching before you go jumping to conclusions!

[timz]

II Phrogs

[N&W's 2-6+6-4] was found quite wanting on grades in excess of 1%?

How wanting? PRR tested the A, but none of us has any idea what tonnage it pulled on what PRR grades, or failed to pull. PRR decided not to bite, but none of us knows why.

[pennytrains]

It'll a bit annoying but it goes away as soon as the moderators realize you're the real deal 

[II Phrogs]

BigJim

 

You only need to look at the performance of the Class A's hauling coal west and east to see that the factor of adhesion was not a problem. 

 

 

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%?

This is the exact reason I have always been a big fan of the Challengers (and why I wanted to know more about their differences across the various roads which used them!). They have always struck me as incredibly well balanced locomotives, being relatively fast and powerful, but not at the expense of adhesion to the rails. They were used by quite a number of railroads and as a result encountered a variety of running environments/conditions. 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.

(unrelated, but I hope I included the quote correctly. I'm still getting used to using the forums and due to the wait time between submitting my reply and having it approved it's taking me some time to figure out the details.)

[BigJim]

II Phrogs

While, at least in the case of the N&W A Class, the 2-6-6-4 seems to produce a fair bit more tractive effort, it also seems to have a surprisingly low adhesive factor, would that not balance things out in the end?

You only need to look at the performance of the Class A's hauling coal west and east to see that the factor of adhesion was not a problem. 

[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,

[timz]

II Phrogs

in the case of the N&W A Class, the 2-6-6-4 seems to produce a fair bit more tractive effort, it also seems to have a surprisingly low adhesive factor

Because it doesn't have that much more weight on drivers, you mean. Well, yes, can't pull hard without weight on drivers.

Saying "surprisingly low adhesive factor" is exactly the same as saying "surprisingly high calculated tractive effort". N&W thought they could get away with largish cylinders; maybe they assumed their engineers were good at coping with slipping. Maybe they were right -- none of us fans knows.

Incidentally, N&W's calculated tractive effort for the A might have been conservative. The A had limited cutoff (75% or so) so in its TE calculation N&W only assumed 77% mean effective pressure.

[II Phrogs]

May I ask what advantages those might be? The only inherent disadvantage of the 4-6-6-4 when compared to the 2-6-6-4 that comes to mind (though I am not in any way an expert on the subject, hence my curiosity) is the shallow firebox of the Challengers, which extends over the rear drivers. While, at least in the case of the N&W A Class, the 2-6-6-4 seems to produce a fair bit more tractive effort, it also seems to have a surprisingly low adhesive factor, would that not balance things out in the end?

[Overmod]

timz

You're probably being too generous when you say there was something in that article to disagree with.

Isn't much question that the evolved 2-6-6-4s had significant advantages over Challengers (the only real 'superiority' being the supposed better guiding of an Adams-style four-wheel engine truck) and by the time of the Alleghenies the issue was beyond much rational 'disagreement'.

Not that I dislike Challengers, mind you.  The D&H engines in particular changed the whole game for D&H operations. 

[timz]

You're probably being too generous when you say there was something in that article to disagree with.