Although investment in hydrogen-fuelled trains is a 'vital' part of the process to improve air quality, it must not be seen as an easy replacement for electrification, according to The Future for Hydrogen Trains in the UK, a new report published by the Institution of Mechanical Engineers. The report considers both 'brown' hydrogen made by steam reforming of non-renewable fossil fuels and the currently more expensive 'green' hydrogen produced by electrolysis using renewable electricity. It says 'the overall efficiency of a hydrogen train is about a third that of an electric train', calculating that hydrogen traction requires 3 kW of electricity to deliver 1 kW of power to the wheel while an electric train needs 1-2 kW. This 'undermines the green credentials of hydrogen trains' if non-renewable sources are used; efficiency becomes less important if renewable energy is used. The report concludes that hydrogen should only be used in places where long-term technical environmental and economic factors make electrification a poor option, for example on remote rural routes. It confirms that hydrogen is not suitable for high speed and freight trains, because the fuel requires a large amount of storage space.