The depot is about 1.5 km south of Three Bridges railway station, on either side of the Brighton Main Line.

Located in the 'fork' between the Brighton Main Line (L&BR 1841), the Arun Valley line (LB&SCR 1848), and the now-closed Three Bridges–Tunbridge Wells line (EGR 1855),[3] the site had historically been used for railway use, having not been built on until railway developments; by 1910 sidings had been built east of the Brighton Main Line, as well as an engine shed and turntable adjacent west of the site;[4] in 2008 the western development area comprised underused sidings and hardstanding with the site east of the mainline including operation sidings, as well as offices; tenants included English Welsh & Scottish Railway, BAM Nuttall, Colas Rail and Balfour Beatty.

In 2008, the Department for Transport commissioned a study into the location of depots for the future Thameslink rolling stock: Network Rail preferred two depots based on an expectation that at times the central area of the Thameslink route would be closed for maintenance outside commercial operational hours, with no workable alternative electrified routes available.

Site offices and warehousing were to be in a 2,857 m2 (30,750 sq ft) three-storey building northwest of the main shed.

[12][13][14] Planning permission for the development was granted in November 2009,[15] but in December the associated Hornsey depot application was blocked by the Secretary of State for Communities and Local Government John Denham; Network Rail submitted revised plans for both sites in 2011, with a smaller Hornsey scheme and an expanded Three Bridges scheme.