Southern Region purpose-designed and built suburban replacement stock was authorised in 1980, when a fleet of 74 four coach units was ordered, classified 510; this subsequently changed to Class 455. The 'new' generation stock, based on the Mk. III loco-hauled profile, followed the London Midland Region's Class 317 units off the BREL York production line, and commenced delivery to the SR at the end of 1982. The Class 455 units reverted to the SR unit policy of the early 1960s by having the powered vehicle in the middle of the formation, but it did not incorporate guard's accommodation. Unit formations being: Driving Trailer Second (DTS), Motor Second (MS), Trailer Second (TS) and DTS. Passenger accommodation (all second class) was provided for 74 in DTS and 84/86 in intermediate vehicles, laid out in the high density 2 + 3 configuration. Coach construction resembled the Class 508 build, but incorporated a revised front end, with a centre gangway and waist height control jumpers. Main reservoir pipes were also fitted in place of automatic connections as used on the 508s. Although the dynamic or rheostatic brake was fitted to the Class 508 units, and proved satisfactory, it could not be used on the Class 455, so conventional EP braking was used. Despite units having end control jumpers, they were incompatible with any other stock. Construction of the first Class 455 commenced late in 1981 at BREL York, but was not completed until the late summer of 1982 when several finished units were transferred to BREL Wolverton Works for modifications. Actual delivery to the SR commenced in December 1982, and the 74 units were all received on the region by April 1984. All the initial order were placed into South Western Division operation, firstly taking over on suburban duties and, later, on outer suburban services.
Upon delivery, sets went to Strawberry Hill CM&EE Depot, near Twickenham, where commissioning was carried out. Unit numbers were 455801-74, but as the SR only used four digits of six figure TOPS numbers, only the last four are actually applied.
In May 1984, the first of the second production run of 43 Class 455 units, No. (45)5701, was delivered, having a revised front end with air horns positioned under the buffer beam, and incorporating a Trailer Second (TS) removed from the Class 508 prior to these sets being transferred to Merseyside. The third and final Class 455 built, Class 455/9 commenced production in early 1985 and was for 20 units (5901-5920), these followed the same body design as the 455/7s and were again allocated to the South Western Division.
From spring 1985, the Class 455/8 fleet commenced operation on the Central Division suburban services being allocated to Selhurst.
When built the fleet were finished in blue/grey livery, NSE colours were applied from the late 1980s. Upon privatisation the majority of the fleet was allocated to South West Trains, with the early members of Class 455/8 allocated to South Central. SWT livery was soon applied and a handful of sets later appeared in Connex South Central colours, after this franchise was taken over by GoVia/SouthCentral in mid-2001 this operators white and green livery was applied.