If there is a suspicion that the metal surfaces have developed rust or any other insulating layer, a solution would be to give the tracks a Brasso (or other metal polish) wipe and quick rinse and dry with water. Also give the train motor wheels a wipe of the same polish.

Also check this YouTube video https://www.youtube.com/watch?v=RCda7voqyUY

If there is a suspicion that the metal surfaces have developed dust and grime or any other insulating layer, a solution would be to give the tracks a Brasso (or other metal polish) wipe and quick rinse and dry with water. Also give the train motor wheels a wipe of the same polish.

Also check this YouTube video https://www.youtube.com/watch?v=RCda7voqyUY

Edit 2: not having Brasso made me try a standard cleaning wet-wipe and after a few wipes it felt smoother (less sticky/rubbing) and guess what? One motor runs perfectly fine, the other stops after a few seconds, I've opened that one up and cleaned the insides (grease got dusty and hard) and it now runs fine without any grease and sounds way better!

A little old issue but still give the tracks a Brasso (or other metal polish) wipe and quick rinse and dry with water. Also give the train motor wheels a wipe of the same polish.

Also check this YouTube video https://www.youtube.com/watch?v=RCda7voqyUY

If there is a suspicion that the metal surfaces have developed rust or any other insulating layer, a solution would be to give the tracks a Brasso (or other metal polish) wipe and quick rinse and dry with water. Also give the train motor wheels a wipe of the same polish.

Also check this YouTube video https://www.youtube.com/watch?v=RCda7voqyUY