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.

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.

Only two units have been preserved.