I found the N-scale train I gave my son several years ago. Put the track together attached the transformer, put the train on the track but it wouldn’t move. When I lifted the rear of the engine off the track, the motor would start running until I put it back on the track. Any suggestions would be appreciated.
First, if you know what brand of loco it is, some of the more knowledgeable folks here may have a better “brand specific” answer.
I would take the shell off and check for a loose connection. I could be a wire that is loose and when the truck is raised it makes contact or it could be a wiper that repositions when picked up.
Just some thoughts,
Richard