The other problem that will occur with steam in particular depend on whether you have plastic or brass. This is especially true of articulated locomotives. Plastic models of Big Boys, Challengers, the USRA 2-8-8-2, etc, will put a swivel in the center of each driver set as well as swivels for the leading and trailing trucks. Brass models a modeled to be as close to prototype as possible including the method of articulation. As a result an Athearn Challenger will work on a 30 inch radius with normal track spacing. We discovered from experience at the modular club that brass Challengers will not work on a 36 inch radius double track on the inside track without the smoke box front hanging past the outside of the outside track on the curves!