ELECTRIC TRAIN SUPPLY

'Electric Train Supply' or 'ETS' is the supply of electricity which powers electrical equipment on a train. It is usually provided by the locomotive, and used to power auxiliary systems on the rolling stock, usually coaches.
Originally, trains hauled by a steam locomotive would be provided with a supply of steam for heating the carriages. When diesel locomotives replaced steam, the steam heating was supplied by a boiler - either oil-fired, or heated by an electric element in the case of diesel-electric locomotives. At this time lighting was powered by batteries which were charged by a dynamo when the train was in motion, and buffet cars would use bottled gas for cooking and water heating.
Later diesels and electric locomotives supplied 'Electric Train Heating' or 'ETH', which was eventually renamed 'Electric Train Supply' or 'ETS' and used to power lighting, heating, ventilation, air conditioning, fans, sockets and kitchen equipment in the train. Each coach has an index relating to the maximum consumption of electricity that that coach could use. The sum of all the indeces must not exceed the index of the locomotive.
Although most locomotive-hauled trains take power directly from the locomotive, there have been examples (mainly in continental Europe) where restaurant cars would take power directly from the overhead wires. On modern Diesel multiple unit trains, such as the Virgin Trains Voyager, the engine mounted below each vehicle provides power for that vehicle.
In the United States, the system is known as Head end power.