These units are built out of upcycled London Underground D78 Stock, built between 1978 and 1981, which were in service on the District line until 2017. To convert the old stock for mainline railway use, it will be necessary to re-use the aluminium bodyshells, traction motors and bogies from the D78 units and fit them out with new diesel engines and interiors. The D-Train units underwent acceptance testing in 2015 and Vivarail pitched them to train operating companies (TOCs), especially those bidding for the Northern franchise. While no orders were received from Northern, Transport for Wales will replace its Pacers with a combination of five D-Trains and trains taken over from other train operators, and other TOCs have ordered D-trains to replace other outdated trains: South Western Railway for the Isle of Wight's Island Line, and West Midlands Trains for the Marston Vale line