Lionel subway slipping on Fastrack

I would check the drive gears and make sure they are meshing properly and that there is no side play with the gears that would cause the gears to not engage correctly with each other. I had that happen on a Williams engine and it was not easily seen until you moved the wheel set with the drive gear side to side. There was just enough play to cause the wheel to slip on occasion and then at other times it would run great. I would also clean all wheels and track again just to be sure there is no oil residue. Another possibility is that one motor is working and the other is not. I hope you are able to solve your problem.