It only gets to 163,000 and that is the simple mode and could not supply enough steam to go very fast. Big boy is the only 4-8-8-4 and that is the biggest wheel arrangement of a steamer that could operate at 70 mph y6b maybe but in compound mode only and then tractive effort is way down.
Only?...Mmmhmm.
UP's 4-8-8-4's would produce higher speeds, and could develop higher HP at higher speeds but they didn't have the starting drawbar pull of a 2100. The thing to remember is that these Y's had boilers the size of a big 4-8-4, and with compound operation it allowed them to squeeze that kind of performance out of it. The 4000's had a humongous boiler with an appetite to match. The higher speed is great but you need to start the train first.
The real point is that each was designed to do a specific job for it's respective road and both performed admirably in that capacity.
