The British Rail Class 421 (or 4Cig) electrical multiple units were built at BR York 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. Towards the end of their life, with the increasing use of newer trains which were equipped with sliding or plug doors, these trains were known commonly as "slam door trains". The fleet's lifespan was 46 years.