Class 252 was the classification allocated to the prototype High Speed Train (HST) unit, numbered 252001.

When originally built, in 1972, the prototype High Speed Train (HST) units were considered to be formed of two locomotives at either end of a rake of carriages.

It was allocated Class 252, and the whole formation was renumbered into a new carriage number series for HST and Advanced Passenger Train vehicles (4xxxx).

[2][3] This entailed a complete restoration of the vehicle and a fitment of a Paxman Valenta engine at Neville Hill TMD.

[4][5][6] The vehicles that made up the prototype High Speed Train are listed below, together with their current identity and use: