Compound Mallet Question

[Anonymous]

Here is an interesting question I haven't been able to come up with an answer for. Perhaps one of the older railheads can help me out.

In a compound mallet, steam first enters the rear smaller chamber to power the rear set of drivers. As it is exhausted, it is passed, under reduced temperature and pressure (some of the energy having been used to move the piston) into the larger forward cylinders, where it is used and then expelled.

Now my question, how does this work? The first part is easy; however once the steam exits the first set of cylinders, it must exert a back pressure on the first cylinder as it enters the front cylinders, reducing the efficiency of the movement of the first piston. Once the steam is in the front cylinder, it pushes the larger cylinder to drive the front drivers, then is expelled against what is minimal pressure.

So simply, at least as I see it, the steam entering the back cylinders moves the drivers and forces the ejected steam into the front cylinder. This seems to be a big waste of energy reducing the efficiency of the locomotive.

So how do they compensate for the back pressure? Is there somekind of reheating of the steam for the second use and a one way valve. Can anyone help me with this one?

[GP40-2]

Short answer: The steam exiting the high pressure cylinders still has some latent energy. Allowing it to expand in volume again (that's why the low pressure cylinders are so large) allows the steam to release this energy.

Sounds good in theory, but didn't work as well as expected in practice. Most compound locomotive were either scrapped of converted into simple expansion locomotives.

Most famous compound was N&W Y6b. Had good low speed tractive effort but was a dog above 30mph. Started losing HP faster than a rock falling off a cliff.

I don't think there is a way to compensate for the backpressure. Compound mallets were used in slow speed service anyway.

[M636C]

In the early years of the last century, before the Schmidt superheater was generally adopted, a number of compound locomotives, particularly those built for Santa Fe, included a reheater which was a separate section of the boiler between the normal barrel and the smokebox. This had larger diameter tubes to reduce the gas flow resistance. In some articulated locomotives with jointed boilers, the whole forward section in front of the joint was a reheater (conveniently close to the forward cylinders).
When the Schmidt superheater was introduced, the extra energy in the steam allowed the elimination of the reheater, since the risk of condensation and cylinder damage was geatly reduced.

Peter

[Overmod]

Keep in mind that compounding can be beneficial, even desirable, as steam pressure rises and full expansion in a single cylinder becomes physically impractical. This was a focus of some American design theory in the mid-20s, producing Baldwin's 60000 demonstrator (now at the Franklin Institute in Philadelphia) as an example.

The 'back pressure' in a steam cylinder is no more a "limitation" on smoothness or hp output than the compression in a diesel is. You size the hp bore and stroke appropriately for the pressure drop (for power) and to give appropriate steam mass flow and expansion to produce comparable power at the LP cylinders.

Modern compounds could be given intermediate balance via a rather simple (pun not intended) method -- by valving an appropriate amount of boiler steam into the LP flow, via an intermediate 'receiver', it was possible to balance the HP and LP piston efforts very precisely, as well as in an equivalent simple-admission engine. I have always associated this with Andre Chapelon, but N&W applied a very similar principle to the later Y6bs (the 'booster valve' discussed by Newton) and AFAIK locomotives so equipped were quite happy running at 40+mph. One can only speculate on whether future development of 'high-speed compounds' would have produced locomotives on which the fuel savings balanced out the higher capital and maintenance costs of the boosting and equalization systems -- certainly low-mass high-strength materials in the running gear would have allowed Mallets to run as fast as freight power normally could on most railroads...

Mallet problems at high speed (as opposed to simple articulateds) usually involved the mass and size of the LP pistons, which worse yet were usually on the hinged forward engine, right where the two-wheel Bissell lead truck acted as a dual-pivot pendulum. "A-hunting we will go!" This, and suspension, are to me much more significant problems than any introduced by differential steam pressure...

[Anonymous]

I'm not sure what it did, but Compounds had something called an Interceptor valve. Some could be run simple for starting, maybe that's what it was for???

Triple expansion steam engines were very common in marine use so you may find a lot more technical information by seraching the web for them. Titanic had 2 of them and the left-over steam had enough pressure left to drive a steam turbine. Most WWII era freighters also used triple expansion steam engines.

[CSSHEGEWISCH]

Robert La Massena's speculative article in TRAINS in 1973 or 1974 addressed some of the problems of compounding. His proposed design for a rigid-frame high-speed compound reduced some of the back-pressure problem.

The French were masters of high-speed compound designs although they were willing to accept higher maintenance down time then any American mechanical officer. French locomotives were tailored more to a specific service than any American design and usually had a pretty low availability rate.

[jchnhtfd]

In principle (and I won't repeat Overmod up there!) compounding can make more complete and efficient use of higher pressure steam; the high pressure cylinders dropping the pressure from the top to some intermediate level, the next set stepping down, and so on. Each set of cylinders is no more (nor less) efficient than it would be if it exhausted at atmospheric and started at atmospheric plus whatever. Things are somewhat complicated by the variable cut-off in the throttle, however. Properly used with dry (superheated) steam, a steam engine running with a partial cut-off is every bit as efficient as a compound running with no or less cut-off.

There were a whole bunch of problems with compounds, though. Overmod mentions the hunting of the front engine on Mallets. This was pretty bad, but later designs of simple articulateds (e.g. Challengers) showed that this could be overcome. Much more serious, however, was the problem on simple engines in getting all the various piston and rod thrusts balanced. The Delaware & Hudson had some fascinating compounds (including a triple expansion) which were highly efficient -- and went through bearings like there was no tomorrow. Maintenance nightmares.

As Csshegewisch notes, the French, who seem to love complicated machinery, were willing to live with the TLC these beasts required -- but remember that some very simple US built Mikes were the engines which saw steam out in France. In the marine setting (some ships went so far as to have quadruple expansion) the maintenance and TLC were available. Still... as soon as turbines became less costly, only the lowest tramps stuck with pistons.

[rrnut282]

Don't forget to think about how the slide valve works. Very little of the time is the valve open to allow the lower pressure steam exhausted by the HP cylinder to push back against the HP cylinder. Thermodynamics says that high (temp/pressure/energy) state steam will naturally seek to go a low(er) state. So, as long as the exhausted steam has had some of its energy expended moving the piston and is lower in temp or pressure, it will seek another outlet (the LP cylinder) rather than work against the higher pressure "raw" steam. (in theory, anyway)

[Anonymous]

GP40-2 says that the N&W Y-6b was a dog above 30 MPH . . .
Sorry, forty, but the Y-6b's horsepower curve didn't drop off THAT rapidly. But the N&W didn't design the Y-6b (and the Y-6b was just the last class built; the Y-5 and Y-6 and Y-6a classes were just as potent as the Y-6b, incorporating all the newer features as they were developed) to produce high speed horsepower. They had the Class A 2-6-6-4 for those applications.

But if a railroad had a topography like N&W's, with the grades and sharp curves N&W had, the Y-6 was the answer. The Y-6 produced DBHP equivalent to the Big Boy and Allegheny at 25 MPH (both those went on to produce more DBHP at higher speeds, of course), but for N&W's grades and curves it was perfect.

The compounding that everyone else, and you, disdain meant that this locomotive could do what it did with a boiler the size of a big 4-8-4.

Now, my question is - that if compounding could work so well for N&W (don't forget that N&W vied to be at the top of the statistical heap as far as gross ton miles per train hour, and gross income carried over to net - with grades and curvature against loaded coal traffic far worse than C&O's) why would some form of compound not have worked as well for C&O and, of course UP? I'm talking about an engine with maybe 63" instead of the Y-6's 58" drivers . . .

On the subject of front end hunting - this needn't have been a problem unless you were going to operate the engine for long distances at speeds in excess of, say, 70 MPH. It wasn't a problem for the N&W 2-6-6-4, and its designers took heart from the high-speed success of the Seaboard 2-6-6-4, which appeared in 1935 and quickly proved itself capable of 60+MPH operation.

But here's another comparison, recently in print, that makes the N&W/C&O/UP deal more telling:

C&O 2-6-6-6 had 67% of its weight on drivers.
Big Boy had 71% of its weight on drivers.
N&W Y-6 had 89.7% of its weight on drivers.
EMD F-unit had 100.0% of its weight on drivers.

This means that the Allegheny's vaunted 6-wheel trailing truck that made it possible to have almost 7500DBHP at 40+MPH, and Big Boy's 4-wheel leading and trailing trucks that helped make it capable of 80MPH speeds, and made it so pretty, were, in mountain service, a liability. Dead weight that had to be dragged up the hill every trip.

So history tells us that if you want super horsepower, or super speed, go with the C&O and UP engines.

If you want to simply make money, maybe follow N&W's lead.

Too bad more didn't. Those old dog Y-6s helped N&W pay out almost two billion dollars in common stock dividends over the 86 years of its existence, they and their predecessor Ys were there for 43 years of it.

Old Timer

[Anonymous]

Old Timer,

I think some eastern steam fans, including the Ed King article in Classic Trains don't really understand what the Western railroads were doing. It's true that the Northerns, Challengers, Big Boys, and Yellowstones were designed to haul a certain sized train over a particular ruling grade at a certain speed without double-heading, but there was a lot more involved in how these locos were used than just dragging tonnage up a short hill.

An important factor was greatly extended engine runs, which meant the loco had to get over all the intermediate sized grades without helpers at timetable speeds. Many of the Western roads have somewhat rollercoaster profiles with long gentle grades both uphill and down. On the Northern Pacific for example, the modern roller-bearing Northerns reduced engine change points from 7 with the old Pacifics to 1 between St Paul and Livingston. At Livingston, they were sent back after a 4 hour turn-around. Challengers on the U.P ran all the way across Wyoming. Others ran on the line from Green River to Portland. They did get helpers or were double-headed on the ruling grade but handled the train the rest of the way.

Because the western grades were often so long, dealing with slow moving helpers like the Y-classes was a problem because what goes up must come down. They either had to be run all the way across the division or run them light back a long way downhill. During WW-II U.P. acquired some compound & non-compound articulateds from the Eastern roads. They tried them as helpers on the Wyoming grades and they ended up tying up the line getting back down. Big Boys and Challengers did run at high speed downhill, the mallets couldn't do that without tearing up the track and themselves. Even with diesels the Western roads try to avoid helpers.

U.P. had tried mallet compound 2-8-8-0''s earlier but they ended up in helper service on the Oregon grades after being simpled and had the original 57" driver tire size increased from 3 to 5 inches. That experiment lead to the 4-12-2 which had 3 cylinders, 67" drivers, 4300 hp, 96,000 TE, and could pull the same trrain as the mallets at double the speed. Of course that engine would be no more suitable on an Eastern coal-drag as the mallets were on the western grades. [:)] [:)]

Also consider that the western roads were hauling a lot more perishables than coal, so getting the trains over the road was more important than dragging every freight car an engine could pull out of the yard. And most of them were quite profitable [:)]

[GP40-2]

The Y6b had a downward spiral of HP above 30 mph. End of story.

The N&W choices of steam locomotives might have helped their profits on the MARGIN, but they, by themselves, did not create all those years of profits. The old N&W made most of its money by hualing coal down hill from the coal fields to the tide water. You could make a tidy profit from this type of operation using just about any type of locomotive. The N&W escaped, for the most part, the de-industrialization and loss of traffic that crippled many of the other eastern roads. If the N&W had the same traffic mix and loss of business as the PRR, it would of suffered the same fate regardless of the Class A, Y, and J locomotives.

UP829 is dead on right. While the N&W locomotives might of been a good choice for their type of operation, they would not work the same way for other railroads. Not only for western roads, but even other eastern roads such as the PRR, NYC, B&O etc who hualed a lot of fast freight. A fleet of plodding Y6b's would be a disaster in that type of operation. To simply say "Follow the N&W lead to make money" concerning the Y6b is down right false. Many other RR's would of lost even MORE money on steam operations using the Y6b.

In the end, it really doesn't matter. The steam locomotive was doomed for many reasons as soon as the first diesel-electric hit the rails in revenue service.

[Anonymous]

Quoth GP40-2: "The old N&W made most of its money by hualing coal down hill from the coal fields to the tide water. You could make a tidy profit from this type of operation using just about any type of locomotive."

Beg to differ, forty. N&W had three mountains to deal with on the main line: Elkhorn, which was a maximum of 2% before 1950, 1.4 after that; Alleghany, which was 9 miles of 1%, and Blue Ridge, which was 1.2%. There were others on major coal-producing branches. Making money hauling coal downhill was a myth, propagated by people who either didn't know what they were talking about or had their own reasons to belittle the N&W's accomplishments. What are your reasons?

At the end of steam, N&W's mines were producing about 5000 cars of coal per day, 5 days per week. A little more than half of that went east, over those three mountains; the rest went west, over a couple of hills between the coal fields and connections at Columbus. How would you deal with that?

King has never said that a Y-6 as configured would be the answer to everybody's questions. But he did say that a compound with, say, 63" drivers, and all the improvements incorporated in the Y-6, might be. And don't forget - the Y-6's boiler was about the same size as a UP 800s; and it weighed 80 tons less than Big Boy.

But nobody except the N&W ever tried. You flatly say that the compounds would be too slow getting back down the mountains, but you're talking about UP and B&O 2-8-8-0s and others that the steam gurus gave up on. You refuse to consider that a higher drivered compound with a low-restriction Low Pressure exhaust system could have done the job, and that if it could, it would have used a lot less fuel and water in so doing.

The Y-6 was designed for N&W. N&W had curvature to cope with, as well as grades.

Do you think Big Boy could have done the job on the N&W? First of all, he'd need an extra joint to get him around N&W's curves.

You talk about the Western engines having to make those long runs, and run fast between the mountains. How long did they run between servicing stops? Did they run farther than the 150 miles from Roanoke to Bristol? How fast did they run between the mountains? Fast enough for the Big Boy to really NEED to be good for 80MPH?

Oh, and if you really believe that N&W made money hauling coal down hill, read King's story about getting them up the hill in April 2004 TRAINS. If you're still not convinced, give me about a week's notice and I'll meet you in Williamson, W. Va. and take you on a little tour.

Old Timer

[GP40-2]

At this point, it's an academic question only. N&W could of made all the "improvements" they wanted to their steam power, but the end result would be the same. A one way trip to the scrap yard.

[Anonymous]

Old Timer

Another thing people often forget in comparing these locos is the difference between eastern & western coal. One reason for those big boilers and 4 wheel training trucks on western engines was to support the jumbo fireboxes needed to burn the stuff. The BTU output was lower, it burned slower and cooler, and with too much draft it would get sucked out of the stack. A U.P Allegheny would need an 8 wheel training truck, an even bigger boiler, and they'd have to bore out the tunnels. It's interesting to speculate what a Yellowstone could have done on eastern coal, but the reality was both U.P. amd N.P. owned their own coal mines and that's what the locos were designed to burn. Besides the Yellowstone and first series Challengers had built-up frames, no roller bearings and the running gear wouldn't take higher speeds.

The later Challengers and Big Boys did have running gear set up and balanced for higher speeds and there are quite a few anecdotal stories of Big Boys going 80 and Northerns hitting 130 on tangents downgrade with a tailwind [:)] It's more likely the running gear was done that way to reduce wear and tear at more typical speeds of 60 & 100 respectively.

The compounds U.P. borrowed were N&W USRA 2-8-8-2's. They quickly ended up in MOW service. The C&O simple 2-8-8-2's fared only slightly better. The compound 2-8-8-0's were U.P.s own, plus the Harriman roads had plenty of experience with the S.P. compound mallets, which were all simpled. Also St Paul to Livingston on the N.P. is 1006 miles, the only service stops were for coal & water.

My point is that N&W wasn't the only road with 'smart' steam people. As mentioned, U.P. owned their own coal mines so there was plenty of motivation to experiment and I think you'll find they did quite a bit including the coal turbines. They didn't build their own in-house but were always heavily involved with the designs. If such a compound would have been the answer to their prayers, they would have considered it. Another factor that killed steam on the western roads was bad water, particularly on the far western districts. Those were the first to get diesels, so the roads found out first hand what they could do. But don't get me wrong - I love steam, Big Boys, Challengers, Alleghenys, Y6bs, Niagras, right down to lowly 280's and yard goats. It's just that I suspect the steam departments at many Class I roads were a whole lot smarter than many steam buffs give them credit for.

[GP40-2]

UP829 said:

"It's interesting to speculate what a Yellowstone could have done on eastern coal"

They did, it was called the B&O EM1.

Fantastic locomotive. They could drag freight up the nearly 3% grades on the West End, and cruise at 70+ mph on flatter sections. The B&O even used them in passenger service on occasion!

They were so well designed and free rolling, 3 men could pu***hem on level track in spite of their 126 foot length and nearly 1.1 million pound weight.

[GP40-2]

"It's just that I suspect the steam departments at many Class I roads were a whole lot smarter than many steam buffs give them credit for."

I'm with you on that, UP829.

I always get a laugh when steam buffs harp on the Pennsylvania RR for their "boring" fleet of 4-6-2 K4 Pacifics and 4-8-2 M1 Mountains. "What a bunch of fools" they say.

Well guess what, the PRR never missed a dividend payment to its shareholders, even during the Great Depression. Can the N&W or any other RR say that???

[Anonymous]

Quoth GP40-2: "Well guess what, the PRR never missed a dividend payment to its shareholders, even during the Great Depression. Can the N&W or any other RR say that???"

Well, yes.

Here's a little information that may interest you:

N&W was reorganized out of bankruptcy in 1896, and began paying dividends on its common stock in 1901. It paid dividends on its common every year, not missing even a quarterly, until its merger with the Southern in 1982.

The total amount paid, from the Annual Reports to the Stockholders, was $1,970,378,285.81. That's $1.97 Billion. With a B.

In 1900-1901, PRR paid $17.9 Million (with an M) for 39% of N&W's common stock, which it held until forced to divest in 1964 because of the N&W's mergers, and the Penn Central merger. Residual payments continued on a decreasing basis until completed in 1974.

Adding up 39% of N&W's annual dividends for each year from 1901 to 1974 means that the PRR received a total of $406,676,041.98 from the N&W. A nice return for a $17.9 million investment, don't you think?

Now, during the years 1930-1940 - that covers the Depression, you may remember - the dividends paid the Pennsy by the N&W totalled $73,859,152.38. Lore has it, but I haven't been able to confirm it, that in one of those depression years the PRR made no money on its own operations, but paid its own stockholders the same amount that N&W paid it that year. So if the PRR "never missed a dividend payment to its shareholders" it has the N&W to thank.

Now, here's another point for you to ponder. After the N&W dividends were withheld after 1964 with just a shrinking stipend to be paid until 1974, how fast did the PC go into the toilet? Post war, 1946 to 1964, N&W's payments to the PRR totalled $193,856, 647.19.

N&W's Annual Reports for its entire corporate existence - 1896-1982 - are available from the N&WHS on CDRom. If you care to confirm this information, go ahead and buy the CDRom and add them up for yourself. If you would blather about N&W's dividends, it would be nice if you knew what you were blathering about.

I'd like to hear from somebody from the C&O or UP with information about what dividends they paid during the same periods.

Remember. That's $1.97 Billion. With a B.

Old Timer

[daveklepper]

Don't forget that some of the old C&O compound Mallets lasted in mine rund service as long as the much younger Allegainies on the main line. Also, didn't some compounds, possibly the N&W's Y-5's and Y-6's, have a valve that allowed supplimenting second-hand steam with direct injection for certain operating conditions? I seem to remember reading that, but it was a long time ago.

[Modelcar]

....The question of compound design engines using steam from one steam cylinder to the next has alwasys been a question in my mind as well as some others in above posts. Couldn't imagine the "used" steam being routed to the larger cylinders to be "used" again without having a bunch of back pressure in the process and losing almost what they gained....I do remember seeing some of the compounds in action and those front cylinder boxes were huge in size...! Interesting question and comments above.

[Modelcar]

...PS: Along the same lines of engines....Referirng to similar design [articulated], engines can anyone talk a bit of the type of sealing that was used in the joint where front and rear engine would pivot on a curve and require a pivot in the steam supply line to the cylinders. Was it some sort of ball joint with some kind of seal or perhaps just a tight fitting steel ball and joint....?

[daveklepper]

The Norfolk and Western and Southern merger was one of the smoothest and least troublesome of any of the major railroads.

[Modelcar]

.....Must have been real railroad people that knew what had to be done and accomplished after the merger.

[Anonymous]

On the contrary, Oldtimer, it is you who is blathering on about a railroad that nobody outside of trailer park in Appalachia even cares about. Even using N&W and the mighty UP in the same sentence is a joke. The UP’s history, national importance, and motive power is legendary. The N&W was nothing more than a glorified conveyer belt of dirty eastern coal from even dirtier mining towns. The mountain grades you glorify are an even bigger joke. You call 9 miles of 1% grade a mountain? No wonder they got away with using those outdated compounds. I think it is you who needs to take some time and come out west and see what real mountain railroading is about. You can blab all the figures you want, but nothing you say will ever put the N&W in the same league as the UP.

In 10 years, the UP will still be around. The NS will be long gone, absorbed into some western road, its history and management forever forgotten.

[tree68]

QUOTE: Originally posted by MR UP

On the contrary, Oldtimer, it is you who is blathering on about a railroad that nobody outside of trailer park in Appalachia even cares about. Even using N&W and the mighty UP in the same sentence is a joke. The UP’s history, national importance, and motive power is legendary. The N&W was nothing more than a glorified conveyer belt of dirty eastern coal from even dirtier mining towns. The mountain grades you glorify are an even bigger joke. You call 9 miles of 1% grade a mountain? No wonder they got away with using those outdated compounds. I think it is you who needs to take some time and come out west and see what real mountain railroading is about. You can blab all the figures you want, but nothing you say will ever put the N&W in the same league as the UP.

In 10 years, the UP will still be around. The NS will be long gone, absorbed into some western road, its history and management forever forgotten.

Every railroad that wasn't forced to live on hand-me-downs bought and/or built the locomotives that would do the job that needed doing. It is useless to try to compare them within the contexts in which they worked. As has been mentioned in other threads, and in the trade press, even a railroad's "signature" locomotive may not have been suitable for the entire railroad. NKP's Berks were restricted to the eastern end of the road. Challengers and Big Boys rarely strayed off their usual haunts.

As for UP still being around in 10 years - that will be decided in the board rooms. If the powers that be decide that the next big merger should create a new railroad, we may be grousing about "Union Southern..." History argues against that route, but you can never tell...

[Anonymous]

Wa'al Hidy, Mr. Big Ol' UP!

Glad I could provide some fodder for your first post!

Now, I wasn't born yesterday; I know a little about rail history and other railroads besides UP. And I know something about hauling tonnage. And I've observed something of railroads over the years that believed their own press clippings.

Of these, the two that share first place are your Big Ol' UP and the Standard of the World PRR. Others include the old C&O and the Wabash.

Now, here's what I want you to do. Take a look at your UP and its steam power and, in your mind, take those Big Boys and Challengers down to that piddling little N&W and put it on their grades and curves and tell it that it had to move 5000 loads of coal every day, five days a week, over that minor-league layout, plus bring enough empties back to load tomorrow, plus haul a good mix of passenger trains and time freights, and see what you could do with it. While you're at it, take your big 800s down there and tell me how they'd do getting a 15-car Pelican out of Marion, Va. on a rainy December night, on its curves and upgrade; or how it would do picking its train back up to track speed after slowing down for the 25MPH curve on Bluefield Mountain.

Then go and add up the Big Ol' Uncle Pete's common stock dividends for the period 1901-1982 and tell all of us how much it was. Then, go back in your Railway Age magazines where they printed the operating statistics of all the railroads, and tell me how badly the Big Ol' UP beat the piddling li'l N&W every time in such things as Gross Ton Miles per Train Hour and operating ratio.

I know, there ain't no urgency hauling coal, but if you don't get the 5000 loads out of the way that they loaded today and get them 5000 empties back to the mines, they won't have anything to load tomorrow and you won't have any place to put it if they do.

Now, Big Ol' UP man, I haven't been out west for quite a while, but I do know how to read. And if your Big Ol' UP has done a bang up job assimilating the CNW and the SP and satisfying all its customers, as well as competing with the BNSF, they've done a heckuva job hiding it from the press.

- your friend, the Hickabilly Trailer Trash Ol' Timer

[Anonymous]

Oh, Mr. Big Ol' UP -

While were on the subject, I heard a rumor the other day - from a reliable source knowledgable about the industry - that the Big Ol' UP was contracting out the haulage of some hot LA-Chicago container traffic, because of capacity and power shortage problems. What's disturbing is that Big Ol' UP was contracting this work out to TRUCKERS.

Now, I'm sure that it's not true, but would you check on it and let us know?

- YF,THTT Ol' Timer

[Overmod]

Guys, the question had, and still has, to do with compound articulated locomotives (I follow DPM and JDI in reserving "Mallet" for compounds).

BTW, my comments on 'hunting' were intended to apply to Mallets with the LP engine in front, with the large piston mass and thrust characteristics causing the problem. A simple articulated, particularly with lightweight running gear, doesn't have as much problem, and I might note that modern practice (including Jabelmann's locomotives) took vertical play out of the hinge joint between the engines, which helps damp out the rolling component of any hunting tendency.

There's no call to po-mouth the N&W just because UP had good late-era steam designs, and not much reason to disparage UP power in revenge. I've always thought that an A class could hold its own anywhere UP ran Challengers... provided the Fabreeka springs were in that two-wheel lead truck, of course... which is not to disparage the Challengers. The contrary situation is easily proved of course... both 3985 and the Pocahontas Division are still there; the only problem would be to bring them together... but the A's did their best work on the west end and on passenger. And are not compounds.

Something to remember is that some of the late French work with 'practical' compounds came too late to influence effective practice in North America -- heck, in a railroad world that rejected Franklin poppet valve gear (a far greater contributor to efficient locomotive operation than compounding, given the relatively low practical steam pressure that can be carried in conventional firetube boilers when long-term maintenance is a recognized operating cost) what chance would proportionally-balanced IP steam injection have?

American Mallets, from Muhlfeld's forward, usually had a simpling valve that allowed HP steam (often through some form of restriction) enter the LP cylinders for starting. You find this kind of arrangement mandatory on something like a von Borries cross-compound (the thing wouldn't start if near a dead center on the HP side!). A point to remember was that high steam demand would call for this valve to be closed long before heavy lateral thrust would be causing stability or track problems, at least with 'typical' boilers and drafting as used on Mallets in their heyday.

The very late improvements on the N&W included front-end design (demonstrably better than anything UP had, btw)... but even so, the "Y7" follow-on was a simple articulated, not a Mallet...

[Anonymous]

Old Timer

Yes a Big Boy could pull one of those coal drags although the curves on the N&W would be a problem. In Ed King's Classic Trains Steam Glory article on the Y's, he shows that a Big Boy has the same drawbar horespower as a modern Y at 25 mph! But IMO the Challenger types were much better locomotoves than the Big Boys. They could and did haul a coal drag at 10 mph, but could also handle a hot shot refer train, a mail and express train, and even a roads passenger trains. They were like the Dash-9's of the steam era.

And lets not forget that the N&W non-compound A's were basically a home grown version of a Challenger. Also the J wasn't a compund 4-4-4-4, so even N&W knew there was something incompatible between the compound design and higher speeds.

I don't claim to know the answer, but I'm beginning to suspect back-pressure in the high pressure cylinders may have been responsible for driver pounding that virtually every road who tried them encountered at speeds over 25-30 mph. I can find no direct references to support that, but the builders and various railroads were very keen on reducing back pressure in the super-steam locos. N.P. in particular had to run more back pressure than they would have liked in the Yellowstones & Challengers because the lightweight coal they used would lift off the grates with too much draft. Company dyno tests and performance data in the book "Northern Pacific Supersteam" show a clear loss of horsepower and they talk about driver box pounding and hard running as a result of back pressure. U.P. went on a virtual quest to reduce back pressure. The large Sweeney stacks on the modernized older power and the double stacks on the Northerns, Challengers, and Big Boys were U.P.'s way to do it without creating so much draft the coal was sucked off the grates.

Unfortunately all of these monster engines were about as big as was practical to build without rebuilding major portions of the railroads. I don't think there was significantly more H.P. or T.E to be gotten and I've also seen plenty of video where a high drivered Northern, Challenger or Big Boy could spin it's wheels with a heavy train, so lower gearing in the form of smaller drivers wouldn't have made much difference. One feature of the diesels that really killed steam was the MU cable. Maybe instead of bigger locos, the steam builders should have figured out how to MU the operation and firing of multiple engines???? Maybe they tried???

[Overmod]

UP829, HP back pressure is a noble thought, but I think misplaced.

Remember that what the HP cylinders "see" is not different from a lower boiler pressure exhausting, say, to 15psi. It's what they're exhausting into that creates some of the problems... but even then, the exhaust is 'cushioned' by the steam pressure in the intermediate pipes. Remember that 'compression' due to early exhaust cutoff is useful for decelerating the mass of the piston and rod, etc.... but also needed so that gas cutting at early opening of the intake valves is minimized.

In any case, back pressure per se isn't likely to be causing problems related to high rod thrust... in most cases I think it would actually reduce their magnitude. Note that the very high peak pressures that were the bane of the Duke of Gloucester (BR 71000) were not explicitly related to back pressure in the sense you mean.

Driver box pounding is addressed through maintenance (and Franklin wedges), and imbalance is inherent in the design of any 2-cylinder double-acting steam locomotive, I think. The quest to reduce back pressure was properly addressed by Chapelon et al., who understood that clearing the exhaust steam properly was a different engineering issue from creating the most 'automatic' draft in the locomotive's front end. A related problem was that you could be 'too good' at optimizing back-pressure reduction; some '40s locomotives (and I recall UP FEF-3s being a notorious example) used so little steam at 80mph in some services that there was insufficient draft for good firing to maintain that steam.

But I think the point of the 'back pressure reduction' keenness was for economy more than performance per se. Note that UP was keen on using exhaust-steam injectors on some of its late power, which don't work well with a back pressure lower than about 10psi gauge (which is about 25psi absolute). What they were "reducing" was the waste in the steam's capability to induce draft -- which is the (somewhat misguided) point of the larger and multiple stacks. There has been some interesting discussion recently concerning the 'best' ways to design a steam-locomotive exhaust and front end (Porta's Kylpor and Lempor designs being examples).

BTW, when you see high-drivered engines spin wheels with a heavy train, it means that smaller drivers MIGHT have made a difference. Very recent article in Trains just argued that evolution of modern steam power would have followed N&W practice (no overbalance with tight lateral compliance on lead and trailing trucks; fairly low drivers; lightweight rods and running gear; good valves and front end) much more than the 80"-drivered 'classical' express locomotive designs. Note that what failed at 115mph on the PRR test plant was the valve drive, and THAT was probably due to unbalanced forces on the valves -- poppets, for example, would have fixed the problem there, even if they introduced other issues elsewhere...

The technical issues of MU control of steamers were solved long ago -- effective ATC for steam locomotives as early as the 1920s essentially contained all the elements required except automatic firing control, and various theoretical designs had solved the latter by about the mid-Thirties. The first problem involves operation with indifferent maintenance (ghastly!); the second problem involves crew acceptance (less than nonexistent... anybody remember how 'sabotage' got its name?); and the third involves legal liability for unattended boilers -- even in states that don't require a licensed person physically next to each boiler.

It's more complicated than you may think to run steam locomotives 'in parallel' via remote. You're constantly adjusting throttle vs. cutoff -- nice as it might be in theory to open the throttle all the way and run the engine entirely via valve adjustments! You really need a computer to accompli***he fine adjustments, but it doesn't have to be a particularly sophisticated one -- an Amplidyne-enabled version of a Valve Pilot is a reasonable first approximation. Then a couple of strain gages in the draft gear front and rear can give you the inputs needed to 'normalize' power...

But would it work reliably day in, day out, regardless of weather, and would it fail safe? Particularly the latter? That was the issue with ATC for over 60 years, and that's really the issue with steam MU control... in a nutshell, the 'big savings' turn out not to be there in the long run, on the bottom line.

[jchnhtfd]

We seem to have gotten a bit off the track here... in places! May I (very gently!) remind folks that, from the engineering standpoint, one defines the problem, in the cases that the N&W and UP boys are going on about, and then determines the solution. In both cases, the problem was how to get a certain type of load from point x to point y most efficiently. The problems, in the two cases above, were radically different (one could say the same thing about the PRR vs. say the Great Northern, also alluded to above!) -- and so the solutions, naturally, were different, too. Both the N&W Y-6b (and its predecessors) and the UP's Challengers and Big Boys were very fine engineering designs, and did what they were intended to do very well indeed. Likewise the GN's superb Northerns and the PRRs equally superb -- but very different -- Pacifics. Don't knock any one of them -- it's a little like saying that a Chevy Corvette is better than a Chevy Silverado (sticking to just one make here, so folks don't get into the Ford/Chevy/Dodge hassle!) because it goes faster, or the other way around, because it hauls more freight. There were very few real dogs in the late steam engine field (there were some real losers earlier, though... but that's another story).

And no one should argue that, in theory, a compound engine (either articulated or on a rigid frame) will be more efficient in terms of fuel used per ton of load hauled. And it is possible to overcome the instability problems of articulated engines with big low pressure pistons. If you have enough money to spend on the design and construction. The problem with compounds is rather fundamental (at least from this PE's benighted standpoint): maintenance and operation. Operation first: to get the best performance out of them, they require a considerable amount of thought and care in operation. You can't just open the throttle and reverse gear and go! As someone noted above, cross-compounds have to be started simple, and shifted over as they gain speed. Most others had a simpling valve to help starting -- they have to be shifted properly. Then the reverser has to be handled just right to get the best cutoff -- in both high and low pressure cylinders -- to ensure that you aren't starving the low pressure cylinders or the high pressure ones -- and this changes constantly with load on the engine. Not that this can't be done, but the question is, can it be done sufficiently consistently to gain the efficiency benefits? Then there is the maintenance problem...

It is worth remembering that one of the great features of North American (and, dare I add, Russian?) style railroading is that the equipment is simple, rugged, and will run forever given pretty simple care. Compounds don't fit that description. Neither, incidentally, do most three cylinder engines (e.g. the UP engine noted above), as the valve gear for most of them was a conjugated motion from the two side valve gears, and any wear in it at all led to horribly unbalanced forces from the three cylinders, with predictably lamentable results.

One of the first things I learned as an apprentice professional engineer -- more decades ago than I care to think about -- was... KISS! (Keep It Simple, Stupid).