These steel bodied air-conditioned outer suburban units were intended to be used on the Waterloo to Reading line, but when available were tested on the SW main line and the Alton branch. In true Southern tradition the styling of the cab front will not win any beauty prizes! Three vehicles in the formation are motored with two Alstom ONIX 800 270kW motors per car. The fourth vehicle is a trailer with the facility for a pantograph installation. Maximum speed is 100mph. They have the facility for both disc and regenerative braking. These vehicles can only work in multiple with other members of their class, thus repeating the inflexibility when class 508 units were first introduced on the Southern Region. The units are formed driving motor composite (DMC(A)), pantograph (not fitted) trailer standard (PTS), motor standard (MS) and driving motor composite (DMC(B)). The driving motor cars are 21.16 metres long and the intermediate cars 19.94. Power operated sliding doors are fitted.

By the summer of 2001 twelve out of the thirty units were finally in operation working six diagrams with SWT hoping to introduce the whole fleet into service by late September 2001. Following the extensive difficulties with introducing these units, upon having its franchise renewed in 2001 SWT demonstrated its confidence in the British rolling stock industry by announcing that the competing German Siemens Desiro range would be their choice for bulk slam door replacement stock with a massive 785 vehicle order (covering main line and suburban options) worth one billion pounds including maintenance and servicing facilities.