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The fleet, which could be on the network in three years, will have the same speed and range as diesel trains but will be almost silent with their only emissions being water released as puffs of steam.

The conversion programme, planned by French manufacturers Alstom, would make Britain a world leader in hydrogen train technology.

The move is in response to the call by Rail Minister Jo Johnson in February for all the UK's 3,900 diesel trains to be scrapped by 2040.

There are growing concerns about pollution from diesel emissions at railway stations. The Rail Safety and Standards Board is studying concentrations of toxic nitrogen dioxide and particulates at London's King's Cross and Edinburgh's Waverley stations and has taken readings from staff exposed to pollution there.