The Steyning Line was a railway branch line that connected the West Sussex market town of Horsham with the port of Shoreham-by-Sea, with connections to Brighton. It was built by the London, Brighton and South Coast Railway, and opened in 1861. It was 20 miles in length. It followed the course of the River Adur for much of its extent and was alternatively known as the Adur Valley Line.

The Arun Valley line, also known as the Mid Sussex line, is part of the Southern- and Thameslink-operated railway services. For the initial part of the route trains follow the Brighton Main Line, and at a junction south of Three Bridges the route turns westwards. It then runs via Crawley, Horsham and Arundel, before meeting the West Coastway line at Arundel Junction. Trains on the Arun Valley line then proceed to either Bognor Regis, Portsmouth & Southsea or Southampton Central.