I bought a small set, Thunderbolt, for my son and after about 6 hours of running the engine is running slower than it did at first. This started after the train derailed and was stuck for a bit. He had moved over to the slot cars and didn't put it back on the rails.I read about lubing the engine but wasn't sure if anything else might cause this.

The engine is an EMD GP40 Diesel.

Thanks