I have recently aquired a Bachmann HO train set(s),they look to be older..I am guessing at the Diesel type....the diesel in question seems to have some kind of short in it..when placed on the track it does not run..lifting the rear truck off the track while keeping the front truck on the track and the diesel will run...place the rear truck back on the track and there are a few sparks from the rear truck and the engine will no longer run....HELP..I don't know a thing about the HO guage..since I collect the O and G scales ???
have you opened up the locomotive to look inside?
is only one truck powered, or are both?
on some of the older bachmann locomotives, only the front truck was powered. these locomotives had the motor mounted on the from=nt truck, and are the infamous pancake motor type. not much to work with there even if you do get it running again.
older locomotives from various makes often would pick up power from one rail with the front truck, and the other rail with the rear one. in this case, the wheelsets on each truck had to be turned a certain way, and it had to be the opposite way from the other truck. if one axle was turned around, the wheels would cause w dead short. turning the offending axle around should cure that problem.
in your case, the sparks from the rear truck point to a dead short, but i am kinda at a loss to explain how this isn't tripping the circuit breaker in your power pack. what type of pack are you using?