In Indian Railways there is a common practice of not switching off diesel locomotives, even when stopped at a station. There are some reasons for it.
1. Many diesel locomotives are old and tend to take an hour or more to get started and thus turning off the locomotive and again turning it on every time is not pragmatic and will result in a lot of delay.
2. Brakes in the trains are fed by a compressor inside the locomotive and the compressors in older trains were mechanical meaning that they rotated due to the rotation of the engine shafts. Therefore, the locomotives were not turned off because the compressors would turn off. Even today, with electrical compressors they power supply must be maintained so the diesel locomotives are not turned off.
3. Locomotives have a lot of electronic controls that consume a lot of power. These controls will need continuous power and the locomotive batteries cannot sustain it for long, therefore needing for the engine to keep running and keep the generator running.
Yet now we have something called as an Auxiliary Power Unit (APU) in diesel locomotives:
This can be allowed to run even when the engine is turned off so that it can supply power for the electronic controls, brake compressor and keep the batteries charged.
An idle diesel locomotive with the engine running will use 20 liters of diesel per hour. Yet, with the APU it will use only 7 liters of diesel per hour.
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