Desired driver size and wheelbase? In decimal inches or mm or standard unit?
Take a look at some of the MicroAce steam, that's how the Atlas 2-6-0 and 4-4-0 got done. Good quality stuff.
Also look at some of the Kato Japanese steam (check out Plaza Japan for these). High quality.
Plaza Japan stocks quite a collection of what's lovingly referred to as 'critter chassis' from various manufacturers, most are ultra tiny and most have 3.5v motors with high speed and low torque. But most have dimensions in the description.
Chris333 and I have been working with Toma drives to do N and Nn3 0-6-0's, that's a honey mechanism, but very, very small.
Old Bachmann and new Bachmann are not the same; they do sell chassis and parts online, that split axle thing on the old stuff is true and notorious but the newer (Kader?) stuff is pretty darn good.