locomotive power problems

Every Christmas I do a large window display where I like to have a G scale train driving around. The train is on a timer and runs on and off through out the day. However my basic Bachmann train is not reliable. The wheels needed cleaning once a day and when the timer turns on the loco often...

Hi, I do have two engines an A and B unit from proto and bachmann respectively. A-unit runs faster than the B unit. would having them put together create any problems? Also how to determine what is the number a cars that any given locomotive can pull.