In 1938, BBC approached the SBB with the idea to build a locomotive that was powered by a gas turbine. The SBB agreed with the limitations that the six-axle locomotive should have a maximum weight of 92 tonnes and a top speed of 110 km/h. The Am 4/6 was built by the SLM with a turbine and electrical equipment from BBC and was ready for testing in 1941, numbered 1101.
Like the Ae 4/6, it had two central powered axles in the frame and one bogie at each end with a powered and a leading axle. The turbine was started by an auxiliary diesel engine and drove a generator that powered all four traction motors. Due to the weight restrictions, the power of the turbine was limited to 2,200 hp. In medium to high power settings, the locomotive reached a total efficiency of 15 to 18 percent, but at lower settings it dropped to below 15 percent.
Trials also took place at the SNCF in 1945 and 1946 and at the Bundesbahn in 1950. In general the performances of the locomotive were favorable, but the efficiency was lower than that of diesel locomotives. Additionally, the SBB didn't have many non-electrified lines at this time and multiple failures would have required a rebuilding. So it was withdrawn in 1954. The only export order for a similar locomotive came from the British GWR in 1946 and was directly delivered as British Railways No. 18000.
To test the multi-current equipment for the new TEE train sets, the Am 4/6 was rebuilt into a three-system electric locomotive between 1958 and 1961. The bogies and traction motors were retained, but it got a transformer for AC service, starting resistors for DC service and a new body. It was put into service in 1964, but most of the time only performed secondary duties due to its low power of only 1,692 kW. After the transformer had been removed due to damage in 1976, the locomotive was only used in the DC network between Geneva and the French border. It was finally withdrawn in 1978.
