The Class 508 EMUs were developed for Merseyside following extensive trials and testing of the 4Pep/2Pep stock built in the early 1970s. Testing of Class 313 stock took place on Merseyside, using 313003 and 313005. However, following planning and building, British Rail decided to divert the 508s to work alongside much older first-generation 4Sub EMUs in the London Waterloo area. Based at Strawberry Hill depot, they soon became very unpopular in the Southern region due to their being non-standard. Once a new build of Class 455 EMUs were complete the 508s were slowly sent to their originally intended home on the Merseyrail network, to work alongside the already well-established fleet of Class 507 EMUs working there since 1978. All 43 508s are now formed of three cars, although they were four