The SBB needed powerful mountain freight locomotives especially for the electrification of the Gotthard railway. The specifications specified a whole range of requirements. Among them was that the entire Gotthard line from Goldau to Chiasso and back was to be completed in 28 hours with a train weighing 860 tonnes. A pusher locomotive was allowed to assist on the steepest parts. In addition, 300 tonnes should be able to be pulled at 50 km/h at 2.6 percent and overloading by 20 percent should be able to be coped with over a period of 15 minutes. In addition, a regenerative brake was part of the mandatory equipment.