One assumes you have cleaned the wheels and the track?
I would definitely go over the track and wheels to make sure that is all clean before starting to disassemble the locomotive. Watch the train from a low angle, can you see any sparks from the wheels to the track? Clean the wheels and track with a low odor mineral spirits and be sure to clean up any pitting you see on the tender wheels. I don't believe the cap is going to improve the running of the trains on the dc current.
Ray