The railway line between Chichester and Havant is set to shut for nine days during the autumn half term for engineering works.
Network Rail will be laying 2.3km of new track and replacing life-expired equipment at a number of level crossings with new more reliable technology.
Engineers will also be carrying out other improvement works at the affected stations when services are not running.
From Monday October 28 to Friday November 1 buses will replace trains between Havant and Chichester, with journeys taking longer than usual.
Meanwhile the line between Barnham and Havant will be closed on the following weekend dates: Saturday October 26 and Sunday October 27, Saturday November 2 and Sunday November 3, Sunday November 10, Sunday November 17,
Saturday November 30 and Sunday December 1.
Buses will also replace trains on these dates.
Throughout the work, trains to London will still run from both Havant (to London Waterloo via South Western Railway) and Bognor Regis (via Southern).
During the weekdays, trains to London will also run from Chichester (via Southern).
Work on the level crossings at Basin Road, Stockbridge Road, Brook Lane and Bosham will result in some diversions on local roads. More information will be available as soon as these are confirmed.
John Halsall, Network Rail’s route managing director for the South East, said: “Over the past year we have invested well over £100million in the south coast railway to provide passengers with more reliable journeys. That work is set to continue with this multi-million pound investment over the autumn half-term between Barnham and Havant.
“There is never a good time to close the railway, but services are significantly quieter over the half-term. Passengers will still be able to travel to London, Brighton or Portsmouth, but may need to use a replacement bus and journeys will take longer.
“Following on from the successful Three Bridges to Brighton blockade, we will work closely with our colleagues at Govia Thameslink Railway to put passengers first, ensuring people are fully aware of the impact on the journey and the alternative options available to them.”
Keith Jipps, GTR’s infrastructure director, added: “We welcome the investment on this part of the route and will work with Network Rail to limit the disruption to passengers during this vital project. An alternative rail replacement bus service will be in place and we’ll ensure that passengers are kept up to date with the options available to them.”