Does the transformer get hot too? Does the motor in the loco seem excessively warm, if you can't hold your finger on it, it is too hot. Are there any hot spots in the track, test by feel. Amazing how an AC powered train will work with track short issues up to a point, the transformer will just pump the amps into the problem. Your unit at 18VAC and 80 VA is about 4 amps of current, so it could be feeding the loco and a track issue too.