An electric-steam locomotive is a steam locomotive that uses electricity to heat the water in the boiler to create steam instead of burning fuel in a firebox. This is a highly unusual type of locomotive that only makes economic sense under specific conditions. Normally, it would be much more efficient to build and use an electric locomotive. However, lack of time and resources (as during wartime), lack of coal or similar fuel, and the presence of relatively cheap and available electricity may make conversion of an existing steam locomotive into an electric-steam locomotive a viable proposition.
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, 16⅔ 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.
A conventional coal-fired or oil-fired steam locomotive may be prepared for service by using an external electric pre-heater. This allows steam to be raised gradually during the night so that the locomotive will be ready for use in the morning.
Modern steam locomotives, such as the rack locomotives of Brienz–Rothorn railway and DLM's modernised class 52.80 locomotive, are fitted with internal electric heaters. This allows keeping the well insulated boiler warm overnight or even to start heating automatically in the early morning.
The electric-steam system is also used in some small-scale model steam locomotives.
- Alfred Moser: Der Dampfbetrieb der schweizerischen Eisenbahnen 1847–1966. 4. nachgeführte Auflage. Birkhäuser, Stuttgart 1967, S. 269. (detailed description in German)
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- Bell, Arthur Morton (1949). Locomotives. 2 (7 ed.). London: Virtue and Co. p. 389. OCLC 67533830.
-  advantages of modern steam as given on the pages of DLM AG, Switzerland.
- Carver, Rex (August 2009). "The Electro-steam story: Kettles on wheels!". The Railway Magazine. 155 (1300): 38–39. ISSN 0033-8923.
- "The Swiss Electric-Steam Locomotives.". Loco locos. 7 Jan 2010. External link in