The nine-day closure of the southern end of the Brighton main line which had been scheduled for the October half-term holiday will now not go ahead.
The change of plan follows a decision by Network Rail to revise the way a major improvement programme is carried out.
Network Rail had agreed two nine-day closures with Govia Thameslink Railway (GTR), which operates Southern, Thameslink and Gatwick Express services.
The aim was to close the lines between Three Bridges and Brighton and Three Bridges and Lewes for two nine-day periods coinciding with the October 2018 and February 2019 school half-terms.
These shutdowns were in addition to a number of weekend closures.
The engineering work is part of a £300 million “government-funded improvement programme to boost reliability for passengers on the Brighton main line and other key routes in the south east”.
It will now be carried out in just one nine-day closure from Saturday 16 February to Sunday 24 February next year, subject to the rail industry’s usual assurance reviews.
The supporting 15 weekend closures from September 2018 to May 2019 will go ahead unchanged.
Some elements of the work as originally planned will now be deferred until the next five-year funding period, starting in April next year.
Network Rail has taken the decision to revise the main body of work in consultation with the Department for Transport (DfT) and GTR.
It said that the change of plan would give passengers more time to plan ahead before the weekday closures in February next year.
And it meant that the rail industry could focus its immediate efforts on embedding the interim timetable from this Sunday (15 July) and delivering a more dependable service.
During the line closures, rail replacement buses will run between Three Bridges and Brighton or between Three Bridges and Lewes.
In addition some services between Brighton and London will be diverted via Littlehampton.
Passengers should allow considerably more time for their journeys during these periods, Network Rail said.
John Halsall, Network Rail’s managing director for the south east route, said: “I know many passengers have had a really tough time since the timetable change in May.
“That’s why I asked for a review of all our planned maintenance and improvement work with GTR to identify any opportunities to postpone or re-plan engineering work to a later date.
“I’m pleased we’ve been able to re-plan the way we’re carrying out this long-overdue upgrade to one of the most unreliable parts of our rail network, meaning passengers will get almost all of the reliability benefits but with significantly less weekday disruption.
“I’d urge passengers to plan ahead and we’ll continue to work closely with the train operators, Transport Focus and passenger groups to make sure the travelling public get the best possible service during the closures.”
Keith Jipps, GTR’s infrastructure director, said: “This route is the most congested and intensively used in the country and Network Rail’s work is essential to give our passengers the reliable, on-time services they want and deserve.
“We’ll be ensuring there are many options for passengers to make their journeys, including alternative transport to other rail stations and with other train operators.
“However, passengers need to know that they will have significantly longer journeys when the railway is closed.”
The engineering work will focus on four Victorian-era tunnels – Balcombe, Clayton, Patcham and Haywards Heath – and the railway which runs through them.
A major programme to stem leaks into the tunnels and provide reliable drainage away from the tracks will take place, while sections of the track, third rail power system and signalling will be replaced or upgraded.
Elsewhere, track will be renewed, sets of points, which enable trains to switch between tracks, will be replaced and fencing will be improved to deter trespassers.
Without this programme of work, reliability on the Brighton main line would deteriorate in the months and years ahead, leading to more delays for passengers travelling between London and the south coast.