The British Rail Class 450 Desiro is a type of third-rail DC electric multiple unit (EMU) passenger train that entered service during 2003.

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 difference in comfort – particularly the higher-density 2+3 seating arrangement used on the Class 450 fleet – has prompted complaints from some passengers affected by the substitution.

[10] Both the Class 444 and 450 fleets are maintained at the purpose-built Northam Traincare Facility in Southampton, which is equipped to perform both preventative and corrective maintenance, deep-cleaning, and overhauls on up to eight units simultaneously.

South Western Railway, as part of its franchise award in 2017, has carried out a refurbishment programme on its Class 450 fleet.

As part of this work, subclass /5 high-capacity units have also been similarly refurbished, and returned to their original numbers, meaning that all Class 450s once again share a common layout.

[18] On 4 March 2024, unit 450092 was derailed between Woking and Surbiton stations when it struck an obstruction on the line whilst travelling at a speed of 90 mph (140 km/h).