For whatever reason, the moving field coil was always a problem in these. When they worked, they worked fine, but when they started to malfunction, it was very difficult to ever get them working again. While the two little drums and contacts were somewhat of a PITA to get in correctly, my nemesis was the actual stepping operation. It would work for a few cycles, then hang up.
MTH ASC Certified Tech, Factory Authorized Lionel Service, ERR Dealer