The British Rail Class 421 (4CIG) electric multiple units were built at BR's Holgate Road carriage works between 1964 and 1972.

Units were built in two batches, and were initially introduced on services on the Brighton Main Line.

The standard units contained only passenger accommodation, and formed the bulk of the fleet.

In 1983, two temporary 8Mig units, given the TOPS classification Class 482 and numbered 2601–2602,[3] were formed while the 4Big fleet underwent asbestos removal.

Therefore, four standard 'Phase 2' 4Cig units, numbers 7401–7404, were reformed with a conventional locomotive-hauled miniature buffet carriage (RMB).

The three former SR division – South-Eastern, South Central and South-Western – all operated 4Cig units, and are dealt with separately here.

South Eastern Trains (SET) operated a small fleet of three 'Phase 1' and 22 'Phase 2' 4Cig units.

All but one of the 'Phase 2' units continued to carry the obsolete Network SouthEast livery.

The three 'Phase 1' units were painted in white undercoat livery, since their use was originally only supposed to be short-term.

1843, lingered on for several months longer than its classmates, eventually being withdrawn in mid-November 2004.

The South Central Division inherited the largest fleet of 4Cig units.

The remaining eleven unit were heavily rebuilt at Eastleigh Works, with compartments removed and opened out, and the buffet carriages withdrawn.

Two units received names – a rarity for slam-door EMU stock.

From 2003 Southern started to replace its slam-door fleet with new Class 377 Electrostar units.

1805 and 1866, were retained beyond August 2005, when slam-door trains lost all diagrammed work.

South West Trains inherited a small fleet of twelve 'Phase 2' 4Cig units, and the larger fleet of 22 "Greyhound" units which had been modified to cut several minutes from journey times on the Portsmouth Direct line with its 1 in 80 gradients by adding a second stage of field weakening to improve performance at higher speeds (17% at 54 mph, 30% at 90 mph).

In 1997 the fleet was augmented with the addition of eight 4Big units from Connex South Central.

The modifications included the removal of the buffet carriages, which were replaced by spare intermediate trailers from mechanically similar 4Cep units.

These trailers were immediately recognisable since although they had the same body profile as the rest of the unit the windows were different and they had InterCity70[jargon] seating.

It was reclassified as Class 421/7, renumbered 1499, and dedicated to the Lymington Branch Line service, where it replaced the previous incumbent unit, no.

The final slam-door service ran on 26 May from London Waterloo to Bournemouth using units 1396, 1398 and 4Vep 3536.

As stated in the relevant timetable, "The slam door trains will be remembered with affection as they pass into history."