Trains are now running between Weymouth and Bournemouth again.
South West Trains has been doing signalling works on the lines since Thursday May 15, causing disruption to passengers.
After two test trains went along the line earlier this evening, the first services ran just after 8pm.
South West trains posted a message on their website, apologising for any disruption to travellers.
It read: “A major programme of signalling renewals has been underway since Thursday 15 May. We have been working to install new equipment and move control of all the signalling systems from traditional signal boxes to a modern signalling centre.
“Regrettably, the process was not completed within the allotted time. Delays to the process had been caused by problems identified during testing of the new signalling modifications.
It added: “Work was an essential part of the investment programme to bring all our network under modern and more reliable signalling control systems.”