If some of your cars are passenger cars, turn each one upside down to look at the wires that go from the wheels to the lightbulbs inside. Sometimes, the wires get frayed, or their insulation gets worn away by the curves having been too small a radius, which wears the insulation off by contact with the edge/flange of the wheels. It happened to me. I had no idea what was wrong and was ready to throw a few passenger cars against a wall; instead, I don't know what made me do it, but I looked at the wires coming from the trucks and saw several were worn through, and every time they came in contact with a wheel at a bend, the train short-circuited.
FrankM
P.S. I fully well realize this phenomenon may have nothing at all to do with what is nettling you, but I just couldn't resist trying to be helpful. If I hadn't tried and later down the long road you had discovered this was your problem, Id have felt badly.
