I am new to the forum.
We have a Spectrum Mallet and a J Class 4-8-4, both run on different layouts and now both present the same problem.
When we apply power, the front light shines but the locos will not move. The wires between the tender and rear of the cab on the J Class have lost their black paint, but that is the only thing I can see which might be the problem.

Any help greatly appreciated.