The British Rail Class 444 Desiro electrical multiple units were built in Germany by Siemens AG from 2003-04. Forty-five of these units were built for express services for South West Trains. They are the flagship of South West Trains.At the dawn of the 21st century, as part of its franchise agreement, South West Trains (SWT) was required to order new trains to replace the old slam-door stock of Classes 411, 412, 421 and 423. Therefore, in April 2001, an order was placed with Siemens AG for 785 vehicles. This order was originally to be split as 100 four-car Class 450/0 outer-suburban units, 32 five-car Class 450/2 inner-suburban units and 45 five-car Class 444 express units.

Units are formed of five cars or coaches, and are numbered in the range 444001-045. Each unit is formed of two outer driving motors, two intermediate trailers, and an intermediate buffet coach. Units have end-gangways, which allows people to move between units when they are used in multiple on services. Trains are limited to a 10-coach length (i.e. two units) as longer trains cannot be accommodated in stations. Indeed, at some stations with particularly short platforms (for example Bentley, in Hampshire), even a single unit of five coaches is too long for the platform, so passengers must board and alight from a single door opened by the guard. The fleet is painted in SWT's express livery of white, with a blue window band, and red/orange swishes at the cab ends.