The work, which will take place during half-term in February 2026, will see buses replace trains between Gravesend, Dartford, Slade Green, Barnehurst, and Crayford.
The closure forms part of a £10 million investment to improve reliability and punctuality at Dartford Junction, one of the busiest in Kent.
David Davidson, chief operating officer for South Eastern Railway, said: “The work at Dartford Junction, where 650 trains pass every day, is a vital investment in the long-term resilience and efficiency at one of the busiest sections on the South Eastern Railway network.
“We know there’s never a good time to do work like this, but we’ve deliberately chosen the half-term to minimise disruption, when schools are on holiday and fewer people are commuting.
“To make the most of the time available, we’ll also carry out additional improvement work at Dartford station.”
Full closures are scheduled from February 14 to February 22, with additional work taking place on February 8 and 9, March 1, and April 5.
During the closure, no South Eastern Railway or Thameslink services will run between Slade Green, Barnehurst, Crayford, and Gravesend via Dartford.
No Thameslink services will operate between Plumstead and Gravesend, though an hourly shuttle will run between Gravesend and Rainham.
All South Eastern Railway Metro routes passing through Dartford will be impacted.
Highspeed services to and from St Pancras via Gravesend will continue to operate.
The engineering work includes renewing ten switches and crossings, laying 400 yards of new rail, and upgrading the conductor rail between Slade Green and Dartford.
Platform adjustments at Dartford station will also be carried out to reduce the stepping distance between trains and the platform.
Mr Davidson added: “We understand this closure will significantly impact travel, and we’re working hard to ensure passengers are fully informed about its impact and the alternative travel arrangements available.”
There will be temporary reductions in peak-time services, particularly affecting the Hayes and Sidcup lines.
Minor impacts are expected on the Bexleyheath, Greenwich, and North Kent lines.
Service reductions will also affect Lewisham, St Johns, New Cross, and London terminals.
Passengers are being urged to make a note of the dates and plan ahead.
More detailed information, including revised train and replacement bus times, will be published in mid-November, which is around 12 weeks before the closure.
Updates will be available on operator websites and the National Rail journey planner.
