Signalling problems are causing delays on rail services between Fareham and Southampton Central.
The problem at Sholing is causing delays of up to 15 minutes on all trains using the route.
Engineers from Network Rail are on site and are assessing the problem.
A spokesman for the Network Rail said: ‘We will be working to fix this problem as soon as possible. We apologise for any delays caused.’
Passengers are advised to check their travel plans at nationalrail.co.uk or by calling 0871 200 4950.