Were these steamers used primarily to pull passengers, freight, or both?
Locomotives with a single axle pony truck, like the 2-8-4, were generally used for freight, with exceptions like 2-6-0’s on branch lines and 2-8-2’s pressed into passenger service during WWII.
They were designed to pull freight at speed, and boy were the designs good!
Probably more than you ever wanted to know about the Berkshire design: http://www.steamlocomotive.com/berkshire/weinstein/