When dealing with old DC traction that cost 10s of thousands to rewind, why risk running in winter if you dont have to. some motors are more sensitive to moisture that others. take the CTA 6000 series. all it takes is a commutator with condensation on it to flash and maybe ground those motors. our long standind practice for moving those cars at the start of the day, or anyother time they been sitting for a while was to run the MG for about 10 minutes before moving the car. let the blower dry out any moisture on the motor.

Did you know the CTA ran the 4000s motors without bottom covers? it was to increase the ventilation on those motors. then again, they also had spare motors and a motor shop to rewind any motors that blew in winter. most museums dont have that luxury.