Es handelt sich um den Prototyp der Class 424, hier mal aus Wikepedia:

The British Rail Class 424 "Networker Classic" electric multiple unit (EMU) was built in 1997 by ADtranz at Derby. It was a prototype vehicle rebuilt from a Class 421 driving trailer vehicle.

The "Networker Classic" concept involved rebuilding Mark 1 design Southern Region EMUs of Classes 411, 421 and 423 to meet current crash-worthiness standards. This involved building a new bodyshell on the existing chassis, but keeping the original electrical and motor equipment. Therefore, the aim was to produce a 'new' unit at one quarter the cost of manufacturing a train from scratch. The rebuilt units would have had a life of at least fifteen years, thus saving cosidering amounts of money when replacing old stock. However, for some reason the project was not successful, and train companies turned to new-build trains of Classes 375, 376, 377, 444, 450, 458, and 460 from various manufacturers.

One vehicle, no. 76112 from 'Phase 1' 4Cig unit 1749 was rebuilt as a prototype in 1997. It was displayed to the public at London Victoria, paired with unrebuilt DT 76747 from 4Big unit 2256, for comparisons purposes.

Since the project was shelved, the vehicle remains stored at Derby. The unrebuilt DT with which it was paired later became part of 4Cig unit 1399, which has since been preserved.