Hydrail is the generic term denoting all forms of rail vehicles, large or small, which use on-board hydrogen as a source of energy to power the traction motors, or the auxiliaries, or both. Hydrail vehicles convert the chemical energy of hydrogen to mechanical energy, either by burning hydrogen in a hydrogen internal combustion engine vehicle, or by reacting hydrogen with oxygen in a fuel cell to run electric motors. Widespread use of hydrogen for fueling rail transportation is a basic element of the proposed hydrogen economy. The term is used extensively by research scholars and technicians around the world.
Hydrail vehicles are usually hybrid vehicles with renewable energy storage, such as batteries or super capacitors, for regenerative braking, improving efficiency and lowering the amount of hydrogen storage required. Potential hydrail applications include all types of rail transport: commuter rail; passenger rail; freight rail; light rail; rail rapid transit; mine railways; industrial railway systems; trams; and special rail rides at parks and museums.