The British Rail Class 421 (or 4Cig) electrical multiple units were built at BR York Works from 1964-1972. 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. These trains were known commonly as "slam-doors".