All forms of rail vehicles, large or small, which use on-board hydrogen fuel as a source of energy to power the traction motors, or the auxiliaries, or both are called Hydrails. Hydrail vehicles use the chemical energy of hydrogen for propulsion, either by burning hydrogen in a hydrogen internal combustion engine or by reacting hydrogen with oxygen in a fuel cell to run electric motors.

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Widespread use of hydrogen for fueling rail transportation is a basic element of the proposed hydrogen economy. Hydrail vehicles are usually hybrid vehicles with renewable energy storage, such as batteries or supercapacitors, for regenerative braking, improving efficiency, and lowering the amount of hydrogen storage required.