If you want to fix a Lionel train, what you probably really need to do is repair a Lionel train motor. There isn’t much to the rest of the train.
The motors tend to be pretty rugged and they’ve held up over the years. Most “repairs” are really more of a clean and service job, not unlike taking your car in for an oil change. Here are some general principles to follow when you clean and service a Lionel motor.