The British Rail Class 450 third-rail DC EMU began service during 2003. They are a part of the Siemens Desiro modular train family. Used for outer suburban services the Class 450 was built with standard and first class accommodation. The unit has a maximum speed of 100 mph (161 km/h).

The Siemens "Desiro UK" family also includes units of Classes 185 350 360 380 and 444. These trains are the most numerous trains in South Western Railways fleet and also the most numerous units in the Desiro UK family.

In April 2001 785 vehicles were ordered by South West Trains in order to complete the replacement of its slam-door rolling stock in accordance with its franchise commitment to do so by 2005 as the slam-door trains were coming to the end of their useful lives and did not meet modern health and safety requirements. Introduction to service was delayed by the required power supply upgrades capable of powering the new trains which feature air conditioning a feature not present in the slam-door trains they replaced. In the December 2004 timetable change the Class 450 Desiro began serving most intended routes although introduction was delayed until June 2005 in some areas. The units are owned by and leased from Angel Trains.

The entire Class 450 series consists of four car multiple units semi permanently formed as DMSO(A)+TCO+TSO+DMSO(B). The Class 450 Desiro trains operate on certain London commuter rail routes as well as outer suburban and regional services from London Waterloo and local services outside of the London area. They are used on all SWR lines.