The British Rail Class 423 (or 4Vep) electrical multiple units were built by BR at York Works from 1967 to 1974, although the MBSOs and TSOs of the first 20, 7701-7720, were constructed at Derby Works. They feature manually opening doors next to every seating row. They were mostly found working outer suburban services in South London, and rural services in Kent and Sussex, up to 2005 when they were finally replaced by Electrostar and Desiro units.