Full disclosure, I'm not an aficionado by any means.   I have an old Bachmann HO set I had as a kid.  it's been boxed up for close to 30 years and decided to take it out to show my kids.   I have a GE U36B Diesel that sorta doesn't run.    Voltmeter shows tracks are getting power but the odd thing is this:   The train has two trucks that holds two sets of wheels each and the drive wheels move only one of the trucks are on the rails, doesn't matter which one is on tracks but as soon as both trucks are on the tracks the wheels stop and voltmeter shows no power on the tracks which to me points to a short somewhere, but since the trucks independently power the wheels I'm not sure where that could be.   Any suggestions on where to look would be appreciated.