UP 4-12-2

[NM_Coot]

Kratville and Ranks say "...were designed to haul mile-long freight trains at passenger train speeds." They also give 4,330 hp at 50 miles per hour; just which hp isn't stated. No graphs or test references. One design requirement was a low axle loading so the third cylinder would help starting what otherwise might be a slippery engine. As others noted, they were adequate and did what they were designed to do, curve straightening not included. Also as noted, the use of a single axle trailing truck was unfortunate. A good, two cylinder 4-10-4 would probably have done as well and not straightened quite so many curves!

[Bsmith]

The main reason for three cylinders was to provide enough cylinder horsepower to take advantage of the adhesion of 12 driving wheels at UP's axle loading of 59,000 lbs., while keeping the cylinders within clearance limits,  reducing piston thrust and crankpin stresses by dividing the drive between two axles, and reducing the dynamic augment that would have resulted from two massive main rods.

As for building a 4-10-4, there's really no need for a 4-wheel leading truck if the prolusion machinery is reduced to 2 cylinders and, therefore, reduced in weight -- compare the ATSF 2-10-4 design, for example, which took advantage of post-1926 advances in metallurgy, boiler pressure limits, and other considerations that allowed the use of two cylinders to deliver even greater horsepower to only 10 driving wheels.

[NM_Coot]

I meant to post this a part of the discussion on the broader 4-12-2 thread but hit the wrong button! Your explanation is correct. The point I was also making was that three cylinders give a more even turning moment and if adhesion might be a problem, that is worth having. I am an absolute fan of the ATSF 2-10-4 engines. Absolutely at home on anything the Operating Department tied on them. Even capable of maintaining scheduled passenger services. I'd take a wager that they cost much less to maintain than the UP Challengers and were just as productive.

[Bsmith]

Yes, That's what I thought you might be suggesting: That a more even turning moment could reduce slipping, but I'm not familiar with any empirical tests showing that this is so (are you?). My understanding of UP's development of, and thinking about, the 4-12-2 series comes mainly from Brian Reed's fascinating monograph on the subject in Vol. 2 of his Locomotives in Profile, titled simply "Union Pacific's 4-12-2s."

You may be right about lower maintenance costs for the ATSF 2-10-4s. It certainly seems reasonable to think that four cylinders, articulated steam pipes, hinged frames, and double reverse linkages would add up to significantly more maintenance work than what would be required by a modern two-cylinder rigid-frame engine. But, I'll bet that if Santa Fe had commissioned some articulateds from Baldwin, they would have been great machines, too.

[daveklepper]

I suspect the Challengers were easier on the track then the Ripley 2-10-4'a great locomotives though they were.   I repeat: The very best non-articulated freight locomotive.