The Horsham, Dorking and Leatherhead Railway (HD&LR) was an early railway company in southern England. It planned to fill in a gap in the network of the London, Brighton and South Coast Railway, shortening the route from London to coastal towns from Littlehampton to Portsmouth. It only obtained Parliamentary authorisation to build from Horsham to Dorking, and it sold its company to the LBSCR, which completed the construction, and itself built the remaining section from Dorking to Leatherhead.
It opened in 1867, and the LBSCR transferred through trains to this shorter route, relieving the congested main line. It was electrified from Leatherhead to Dorking in 1925 as part of the Southern Railway's outer suburban electrification scheme, and in 1938 the rest of the line was electrified, completing the route to the coast. A new regular interval service of fast trains was commercially successful.
The through fast trains were diverted to the Three Bridges route from 1978, and the line now carries only local passenger trains.