Much of the Arun Valley line from Arundel to Three Bridges will be affected - and there will be no trains at all running through Horsham for a nine-day period.
It is part of a multi-million pound upgrade which will affect Southern and Thameslink services.
As well as the nine day period in August when there will be no trains through Horsham, there will also be five weekends of engineering closures in July, September and October.
From Saturday August 14 to Sunday August 22, buses will replace trains along much of the Arun Valley line, from Arundel to Three Bridges including Horsham and Crawley, and the Mole Valley line between Horsham and Dorking.
Additionally, there will be three weekends of engineering closures ahead of the main work and these will take place on Sunday July 11, Saturday July 17 to Sunday July 18, and Saturday July 31 to Sunday August 1.
Following the main closure, there will be a further two weekend closures on Saturday September 4 to Sunday September 5 and Sunday October 3.
Network Rail Southern region’s investment director Paul Harwood said: “We never take the decision to close the railway for extended periods lightly and would like to thank passengers for their patience as we work to address historic underinvestment in the region’s railway and enhance the capability of the line.
“This investment was originally planned for several bank holiday closures, supported by a long series of weekend closures over two to three years. By doing the work over nine consecutive days and two weekends, the overall disruption to passengers can be significantly reduced, while fast-tracking the delivery of the reliability, safety and connectivity benefits of the project.
“Recognising the importance of the railway to the leisure and tourism industry, we’ll ensure that alternative routes are fully open on these dates, and we’ve timed the work to avoid Brighton Pride.
“Over the next months, we will continue to work closely with Southern, Thameslink and South Western Railway colleagues, as well as local stakeholders to ensure passengers are made aware of the changes to their journeys and the alternative options to keep them moving.”
Chris Fowler, customer services director for Southern, said: “Network Rail have worked closely with us to plan this vital work so that our customers’ journeys are disrupted as little as possible.
“The upgrade will improve the reliability of services on the Arun Valley line, and I thank customers in advance for their patience while work is underway in the summer.
“We’ll be giving more detailed advice about alternative travel options over the coming months.”
Network Rail says that engineers will replace 1980s track and infrastructure, “with new, more reliable equipment that will improve journeys for customers on the two routes, as well as the wider region by reducing delays.”
It warns that passengers travelling on the dates affected will need to allow considerably more time for their journeys and should expect to use either diverted trains via longer routes or a replacement bus or coach to connect with rail services to the south coast.
Work has also been timed to avoid disrupting Brighton Pride at the start of August and other routes including the Brighton Main Line, will be open for travel.
Services on longer distance coastal journeys to Chichester, Littlehampton, Bognor Regis and Portsmouth will still be running, but will go via an alternative route using the Brighton Main Line, meaning journeys will take longer.
Crawley station will remain open on weekdays during the nine-day period but will be closed on all affected weekends.
The work will involve the complete renewal of nine sets of old switches and crossings - the movable sections of track that guide trains from one track to another and allow them to cross paths.
The existing equipment has been in place since the 1980s and is said to be unreliable. The new equipment will reduce faults and delays to passengers, says Network Rail.
A section of rail, sleepers and ballast will be renewed, and track drainage replaced at Horsham Station.
The work will also enable the upgrade of a level crossing at Parsonage Road, Horsham, to a full barrier CCTV crossing to improve safety, and recontrol of Littlehaven signal box to the Three Bridges Rail Operating Centre in June 2022.
To give passengers additional benefits Network Rail says it and train operators are looking at options to refurbish stations when they are closed.
More about the project is available at www.networkrail.co.uk/ArunValley.