A track defect is causing a delay for trains running on the London to Brighton railway line.
Network Rail confirmed engineering works are taking place today (April 13) outside Patcham Tunnel.
A spokesman said: “We have put a 5mph speed restriction in place, which is causing delays, and we are working on a plan for getting it fixed. We apologise for the delays.”
Southern Railway expects delays of up to 20 minutes.
A spokesman for Network Rail added: “Repairs are likely to take at least three hours and so they will be carried out overnight, when trains are not running.
“Safety is our priority and this decision means we can keep trains running and take people home tonight.”
The defect is a broken insulated rail joiner that links tracks together.