Switzerland has no natural reserves of coal, but the mountainous region offers plentiful, and cheap, hydroelectricity. Thus 2191 km of SBB lines (73.6% of the network) were under the wires at the outbreak of the Second World War, whilst the price of imported (German) coal kept rising.

In an attempt to save on coal the Swiss Federal Railways fitted two small 0-6-0 steam shunters of class E 3/3 with a pantograph. Power was taken from overhead lines (15 kV, 16+2⁄3 Hz), and fed to heating elements, via two transformers rated together at 480 kW. The modified E 3/3 8521 was brought into use on 13 January 1943; 8522 followed on 11 February 1943. They could run up to 20 minutes without power supply, like a fireless locomotive, once the boiler had been charged to full pressure. The firebox was retained, usually keeping hot embers, with a classic fire for longer operation on non-electrified tracks. The water circulation pump, the control circuit and the lighting were powered by a battery that was charged from a rectifier fed by one of the transformers.

The system was capable of producing about 300 kg of steam per hour at 12 atm pressure. It weighed about 7 tons (increasing the weight of the locomotive from 35 to 42 tons) and allowed a saving of 700–1200 kg of coal per working day. Bringing the locomotive to pressure took about one hour.

The electric heaters were removed in 1951 from locomotive 8521 and in 1953 from 8522. As of 2013, locomotive E 3/3 8522 is still in (museum) service on the Sursee–Triengen railway as an ordinary steam engine with no electric heating.