The British Rail Class 377 Electrostar is a British dual-voltage electric multiple unit passenger train (EMU) built by Bombardier Transportation on its Electrostar platform at Derby Litchurch Lane Works from 2001 to 2014. The trains work suburban services in South London, and main-line commuter services to Sussex, Surrey, Kent and the South Coast, on which they replaced 4Cig and 4Vep slam-door stock that was more than 40 years old and did not meet modern health and safety requirements. Built in the early 2000s, the units had a troubled introduction. Being fully air-conditioned, their higher power consumption compared to the trains they replaced led to major upgrades being required to the 750 V DC third-rail power supply used in the former Southern region.[3] The collapse of Railtrack following the Hatfield accident further delayed this upgrade work, and the new stock did not enter squadron service until 2003.