Class 799 "HydroFlex"

The British Rail Class 799 HydroFlex is a one-off tri-mode multiple unit prototype adapted from a Class 319 electric multiple unit. It will be used on the Regional service in the future. It is unknown how much points or Robux it will cost.

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In September 2018, Porterbrook announced it would develop a hydrogen fuel cell demonstrator in partnership with the University of Birmingham's Centre for Railway Research & Education. In December 2018, Porterbrook procured an FCveloCity fuel cell unit from Ballard Power Systems.

The unit was converted from Class 319 EMU 319001 that had last been used by Govia Thameslink Railway. It was unveiled on 20 June 2019.

In June 2019, it was announced that approval for mainline testing of the unit (branded as HydroFLEX) had been granted after a period of demonstration at Rail Live 2019.

The first mainline testing occurred in September 2020, reaching 50 mph through Warwickshire. The next phase of the development of this prototype is move the hydrogen tanks and fuel cell from one of the carriages and suspend them beneath the train.