When I run one of my (steam) locomotives on various different routes in DCC (easy) mode, the speed that each "notch" of the throttle provides are different. For example, if I set the dial at 40%, in one route the engine does 8 mph, in another it does 11 mph, and in a third one it does 14 mph (14 being the speed that it always ran at with that amount of power previously on these routes, I just noticed this problem recently). There is no difference in consist (these numbers are all with full tender loads and no consist). What could possibly be causing this? Is it a problem with the engine or the routes?