We bought it brand new.

My son and I are discovering model trains together. To start, we got a Bachmann HO Pacific Flyer set, which has been working fine. Yesterday his grandma got him an EZ Track expander set, so we've added on a little bit, and we also got a new diesel engine. Suddenly last night we started having a problem with the Pacific Flyer engine - it will only move backwards. We've tried flipping it both directions on the track, and if we try to make it go in whichever direction is forward relative to the engine, the light doesn't come on and it doesn't move. If we switch the direction of the controller, then the engine moves backward just fine (no light comes but but I don't remember if it lit up going backward or not). The new diesel engine runs forward and backward totally fine in all configurations.

Any suggestions on what might be wrong with this engine, or where to start troubleshooting?