Horsham, Dorking and Leatherhead Railway

The Horsham, Dorking and Leatherhead Railway (HDLR) was an early railway company in southern England. It completed earlier plans for a line through the Mole Valley, joining several important towns across the North Downs. It was promoted and later acquired by the London, Brighton and South Coast Railway (LBSCR), and became an essential part of the route later known as the Portsmouth Line.
History.
From the 1850s the LBSCR had been developing lines in Surrey and West Sussex to block its major competitor, the London and South Western Railway (LSWR), and improve its access to Portsmouth. To this end it had built the Arun Valley, or Mid-Sussex, Line which by 1863 ran from the Brighton mainline at via to Arundel Junction on the West Coastway Line. Further north, the LBSCR and LSWR had jointly acquired the Epsom and Leatherhead Railway (ELR), providing a link from the margins of suburban London to the Mole Valley gap at . The ELR was the latest in a series of schemes to bring a railway across the North Downs that began with the Wimbledon and Dorking Railway.
The HDLR was incorporated in 1862 to connect the Arun Valley line more directly to London. The route originally authorised ran from Horsham to a junction with the South Eastern Railway's Reading, Guildford & Reigate Rly (RGRR), joining it east of Boxhill station (now ) in the Guildford direction. The intention had been to tap the SER's Dorking traffic, but although built this link was never used for regular services. Instead in 1863 the LBSCR obtained powers for a continuation from the HDLR through the Mole Valley to Leatherhead, with a new station north of the RGRR.
The LBSCR absorbed the HDLR in 1864, before construction was complete. The Dorking to Leatherhead extension was in fact opened first, on 11 Mar 1867, with one intermediate station at "Box Hill" (now ). At Leatherhead it joined the ELR a little to the east of the existing station, which became a terminus used only by LSWR services. A new LBSCR Leatherhead station was built west of the junction, across the road from the ELR/LSWR station. Horsham to Dorking opened on 1 May the same year, having been delayed by construction difficulties with Betchworth Tunnel. There were intermediate stations at , and .
Services.
The HDLR was a key link in the Portsmouth Line, and became the usual route from Victoria to Chichester and Portsmouth for the next 100 years. It also cemented Horsham as the railway hub of West Sussex, with routes to: Guildford; Chichester & Portsmouth; Brighton via Shoreham; London via Three Bridges; and now London via Dorking and Epsom.
The entire line and all stations have remained in use since opening (with some renaming), although the LBSCR-to-SER Boxhill spur was lifted in 1946, but it is no longer the normal route for Victoria to Portsmouth fast services. Since 1978 these have run via Three Bridges in order to serve Gatwick Airport.
The principal services as of 2020 are:
Features.
The HD&LR required some significant engineering works for a relatively short line. Between Leatherhead and Dorking it passes through the North Downs via the Mole Valley, but to maintain a reasonable alignment alongside the winding River Mole it uses:
Notably Mickleham Tunnel exits directly onto a three-arch viaduct, a feature not uncommon in the Alps but somewhat rarer in the UK.