Starting in 1946, 38 M1a engines were upgraded and reclassified M1b and put to work in heavy freight service. Feature powerful five-pole can motor with flywheel, diecast loco and tender chassis, detailed underbody, plated bell and whistle, cab interior, operating headlight, magnetic knuckle coupler on tender, RP-25 contour on drivers and wheels, and Quantum sound system. Locos can be operated on Code 70, 83 and 100 rail, and on minimum 18" curves.