Network Rail is to upgrade Penge tunnel, between Brixton and Beckenham Junction from 23-31 July.
Almost 4,000 metres of track will be replaced in the £4.5 million project, some of which dates back to the early 1970s.
The 1.25-mile-long tunnel was reputedly disliked by Queen Victoria, leading – it is said – to the construction of the Catford loop so she could avoid the tunnel.