Museum at Crich, Derbyshire uses 600 V DC. This voltage was chosen for maximum compatibility with its historic fleet of trams as well as more modern units. The Wirral Tramway uses 550VDC. The Seaton Tramway uses 120VDC. Existing systems - third and fourth rails National Rail: 650 V - 750 V DC, third rail (top contact) Southern Electric The extensive southern third rail electric network covers South London and the southern counties of Dorset, Hampshire, Sussex, Surrey and Kent. The London and South Western Railway (L&SWR) third-rail system at 660VDC began before World War I from London Waterloo to suburban destinations. The Southern Railway was formed in the 1923 grouping; it adopted the L&SWR system, and by 1929 the London, Brighton and South Coast Railway (LB&SCR) suburban overhead network was replaced by third rail. The South Eastern Main Line was electrified at 600V, later upgraded to 750VDC. The third rail extended throughout most South London lines out of all its London termini. Throughout the 1930s, there was much main line electrification, including the Brighton Main Line (including East, West Coastways and related routes in 1932–1933), the Portsmouth Direct line (4 July 1937) and to Maidstone and Gillingham (1939). After World War II, electrification was soon resumed in the newly nationalised British Railways' Southern Region. The BR 1955 Modernisation Plan included the two-stage "Kent Coast Electrification". The Chatham Main Line was completed, followed by the South Eastern Main Line and related lines. The voltage was raised from 660V to 750V. Since then, all electrification has used 750V; lines electrified before then remain at 660V. Attention then switched to the neglected former L&SWR area (then the South Western Division). The South West Main Line (SWML) to Southampton Central and was electrified in 1967 and to in 1988. During sectorisation in the 1980s, Network SouthEast conducted extensive infill electrification. The Snow Hill tunnel was reopened, enabling Thameslink. The Hastings Line, Eastleigh–Fareham line and the Oxted line (East Grinstead branch) were electrified. This left only a few lines unelectrified: the West of England line, the Wessex Main Line, the North Downs Line, the Oxted line (Uckfield branch), the Marshlink line and the Eastleigh–Romsey line. Merseyrail Two lines of the Merseyrail network; the Northern line and the Wirral line use 750V DC third rail (see Suburban electrification of the London, Midland and Scottish Railway for its history). Island Line (Isle of Wight) The single remaining