Primarily the feedwater heater. There is also little relationship between a Y and an A, other the the articulation. More similarities, including the boiler design, between an A and a J.

Gary,

You say primarily, so what else? Hypothetically, if one were to take the 6a in St Louis and modernize it to become a 6b what else would have to be done?


Thanks, Jason

Gary can probably answer better than I can, but I'll try:

By the time the N&W got done "tweaking" them, there was virtually no difference between the Y6 and Y6a engines. The Y6b had a different feedwater heater, which resulted in a slightly different smokebox size and configuration, which included a slightly tilted stack. The Y6/Y6a had two air compressors on the engineer's side to displace the weight of the feedwater heater on the fireman's side, whereas the Y6b had one compressor on each side due to the feedwater heater in the smokebox.

Additionally, the N&W motive power dept. upgraded their Y5 engines to the same mechanical standards as the Y6/Y6a/Y6b, so that all Y5/Y6 engines were essentially the same in drawbar, horsepower, and mechanical standards. This included roller bearings and some new frames for the Y5's. In a "trickle down" effect, some of the older Y3 (USRA) engines got the better discarded frames from the Y5s to replace their own, and the front ends of those Y3s looked much like a Y5/Y6 afterwards.

There was a Y7 on the drawing board when Stuart Sanders threw in the towel and bought diesels. The Y7 was an improvement on the 2-8-8-2, but it was a "simple" engine as opposed to "compound". It looked like a cross between a Y and the A, with the two air compressors mounted on the smokebox because the boiler was so big. It was meant to combine the pull of the Y with the speed of the A, with four high pressure cylinders and tall drivers.

Everybody talks about the Y6s like they were 15MPH drag engines. I've seen old movies of them bookin right along. Lots of engines got thrown into the "drag" bin when they were used in some pretty fast trains, such as the IC 2-10-2s and Q 2-10-4s. Balanced right, they ran the heck out of them.

Frisco1522 is correct. The N&W Y5/Y6 class spent the bulk of their time at 15mph, but could do 50mph if needed.

The main visible difference is the Y6a has a Worthington BL type feedwater heater mounted on the fireman's side of the boiler, while the Y6b's have Worthington SA types mounted at the front top of the smokebox. There were other less-visible detail differences.

IIRC, by the mid-1950's the Y6a's were pretty much modified to match the Y6b's apart from the feedwater heaters.