The Mid-Sussex railways were a group of English railway companies that together formed what became the Mid-Sussex line, from Three Bridges through Horsham to Littlehampton, in southern England. After 1938 the Southern Railway operated a regular electric train service ran from London to Bognor Regis and Portsmouth using the marketing brand "Mid-Sussex Line", leading to an informal consensus.[1][2][3][4] The Mid-Sussex Railway company ran from Horsham to Petworth, and the Mid-Sussex Junction line of the London, Brighton and South Coast Railway (LBSCR) extended from the Petworth line to Littlehampton. The Three Bridges to Horsham branch of the LBSCR was at first the sole access from the north to the Mid-Sussex railways, although a line from Leatherhead was used later.
The through route was opened in a series of stages with only a vague strategic objective, but in the last quarter of the nineteenth century it became the shortest route of the LBSCR to south coast locations between Littlehampton and Portsmouth. The through route opened in 1867, and electrification was completed in 1938.
The route continues in use as a main line, with passenger train operation using the brand name Arun Valley line south of Horsham. The final part of the line to Petworth has been closed.