If a wagon has a metal underframe and soldered metal couplings, then electrically, an entire train is at the same potential. Now, if a wheel tyre happens to rub on an underframe component, the entire train is now connected to that wheel (and rail).
This can have lots of unwanted consequences;
- another wheel might also rub elsewhere in the train on the opposite side (short circuit, perhaps DCC command station shuts down, or maybe it puts 5amps through the train)
- if two items have decoders wired such that the body of the vehicle is connected to blue (common supply), but that blue might not like receiving supply from the rail on the "wrong" side.
- even without wheels touching, could have two decoders cross-connected, which creates an interesting current path through two decoders (consequences unknown).
Ways round the problem:
Insulate your couplings !!! Don't solder direct onto a plate on the wagon, but fit a piece of plastic or double sided printed circuit board.
Don't connect electrically to the body of a vehicle. Hard to do avoid, but try to minimise.