The Arun Valley Line is a branch of the Brighton Main Line, leaving the line just after Three Bridges. It then runs via Crawley, Horsham (where it meets the Sutton & Mole Valley lines) and Arundel before meeting the West Coastway Line.

The line is currently being upgraded to provide an alternate route to the south in the event of the Brighton Mainline being closed for engineering works or due to an incident.