700E posted:A possible reason why Berkshires were common on Eastern roads but not on Western roads is that clearances tended to be tighter on the Eastern roads. For example, locomotives on the New York Central had to be less than 15'3" in height. (This is why the NYC / P&LE Class A-2a Berkshires had unusually short smoke stacks.) On railroads that had less strict clearance requirements, it may be that something other than the 2-8-4 arrangement made more sense.
The 15'3" clearance was imposed by the tunnels on the Hudson Division, east of Albany (the earliest constructed portion of the system). Central's motive power was built to operate through to Harmon in both passenger and freight service, hence the limited height on the Hudsons, Mohawks and Niagaras. Clearances were more generous, west of Albany. The routing for Union Pacific's Big Boys, Challengers and 800s, for example, from ALCo in Schenectady to Chicago was by New York Central, except for a short bit on D&H departing the plant itself. High and wide loads to the New York City area followed the B&A to Chatham, the Harlem Division to Brewster and the Putnam Division to High Bridge, thereby bypassing the Hudson and lower Harlem line restrictions.
