Put a 2" long piece of painter's or masking tape on the center rail of your test track where the front roller pickup rests and apply power. If the engine moves, then the rear pickup is good. If not, you may have a loose or bad wiring, short, etc. in the rear roller.
Reverse the engine so the rear pickup now rests on the tape and re-apply power and see if it moves with same, above, results for the front roller.
If both pickups are good, the problem lies elsewhere.
