The southern end of the Brighton Main Line is about to be shut for nine days during the school half term.
More than 200 rail replacement buses will be in operation while work on the £67 million upgrade takes place from Saturday (February, 16) to Sunday (24).
There will be no direct trains between Three Bridges and Brighton or between Three Bridges and Lewes.
According to rail bosses, it will be the biggest refurbishment of the line between Three Bridges and Brighton in more than 30 years.
Improvement work will focus on four Victorian-era tunnels in Balcombe, Clayton, Patcham and Haywards Heath.
Network Rail and Govia Thameslink said this is to stop leaks and drainage issues which have been responsible for some of the significant delays in the past.
Other areas of work will focus on renewing and upgrading track junctions and signalling.
New technology will be introduced to detect potential problems before they occur.
John Halsall, managing director, Network Rail South East route, said:
“This work is absolutely critical as this stretch is one of the most unreliable parts of the network, accounting for half the delays to trains arriving from the south into central London.
“We know it’s never a good time to close the railway, but the only alternative would have been to close the line for 84 weekends over the course of two years.
“Doing the work over this nine-day period means we can achieve so much more and deliver the punctuality and reliability benefits to passengers much sooner, plus the railway is much quieter during the school holidays.”
Infrastructure Director for Govia Thameslink Railway is urging passengers to either consider alternative routes, work from home or travel outside rush hours.
Brighton services will be diverted along the coast to London, via Littlehampton, adding an hour to journeys.