The works entail a multi-million-pound sheme to upgrade 1980s railway equipment along with an extra £5 million being earmarked to improve stations, track and signalling.
The work will be carried out during a nine-day rail closure from Saturday August 14 to Sunday August 22 and several weekends between July and October.
Stations along the Arun Valley line will be deep cleaned and refurbished, track circuits replaced and the renewal of sections of track at Pulborough, Warnham and Barns Green.
Nine sets of switches and crossings at Horsham - the movable sections of track that guide trains from one track to another - will be renewed and a new crossover on the approaches to Horsham Station will be installed.
Drainage work to help reduce the risk of flooding on the line will also be completed.
There will also be three weekends of engineering line closures in July and a further two weekend closures in September and October.
All trains via Horsham will be affected on Sunday July 11, Saturday July 17, Sunday July 18, Saturday July 31 and Sunday August 1.
Additionally Horsham trains will be affected on Saturday September 4 and Sunday September 5, along with Sunday October 3.
Network Rail Southern region’s investment director Paul Harwood said: “Network Rail is committed to making journeys better for passengers and this project will help to improve the reliability of services on the Arun Valley line.
“By doing the work over a nine-day closure, we can not only get essential work done more quickly with less overall disruption to passengers but deliver more improvement work than traditional weekend access approaches allow.”
Chris Fowler, Southern’s customer services director, said: “Network Rail’s work programme is absolutely essential to improve reliability for customers on our network.
“We’ll be running replacement bus services, and trains will continue to run along the West Sussex coastal route and between Brighton and London.
“That does mean journeys will take considerably longer than normal, so please make a note of these dates, and check journey planners for details of the alternative arrangements nearer the time.”