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. Later units were introduced on services to Portsmouth. These units replaced older Southern Railway-designed units, such as the 5Bel "Brighton Belle" units, and 4Cor units. The fleet's lifespan was 46 years. Standard units The standard units contained only passenger accommodation, and formed the bulk of the fleet. They were unusual in that all four traction motors were mounted on one non-driving motor coach. Units consisted of two driving trailers, sandwiching the non-driving motor coach and an intermediate trailer. Units were built in two batches. 'Phase 1' units were built from 1963 to 1966 for the Brighton Line, and were numbered 7301-7336. These were followed in 1970-72 by the 'Phase 2' units, built primarily for the Portsmouth line, numbered 7337-7438. 4Big units The 4Big units were similar to the standard units, but contained a buffet car in place of the intermediate trailer. These units were also built in two batches. 'Phase 1' units were built in 1965/66, and were numbered 7031-48. 'Phase 2' units were built in 1970, and were numbered 7049-7058. The 4Big fleet were initially classified as Class 420 by British Rail under the TOPS system introduced in 1968, and then Class 422. Vehicle numbering The numbering of individual vehicles and details of when units were built are shown in the table below. Refurbishment Units were facelifted between 1986 and 1993. 8Mig units In 1983, two temporary 8Mig units, given the TOPS classification Class 482 and numbered 2601–2602, were formed while the 4Big fleet underwent asbestos removal. They were used on the Portsmouth Direct line. Therefore, four standard 'Phase 2' 4Cig units, numbers 7401–7404, were reformed with a conventional locomotive-hauled miniature buffet carriage (RMB). Each 8Mig unit was formed by marshalling the RMB coach in between two units, one of which was reduced to three carriages, with the removal of the intermediate trailer (TSO). Thus in effect each 8Mig unit was effectively formed of 4Cig + RMB + 3Cig. The formations of the units are listed below: These units were only used during the summer of 1983, after which enough asbestos-free 4Big units were available. The two 8Mig units were disbanded and the four 4Cig unit reformed. The 4Cig units have