"Santa" gave my son a HO scale Bachmann EMD F9 Diesel trainset over 15 years ago.  I recently took it out and am thinking of building a set for it.  I have put the track together, hooked up the transformer and put the engine on to make sure it still works. 

Nothing happens until I lift the back of the engine off the tracks and then the motor runs I put it on the tracks and it stops. I have switched the wire connecting the transformer to the track and turned the plug in the socket.  I have checked the track to make sure it is connected and it is.

What do I need to do? Help me please!