Thameslink has warned that its services could be disrupted into the afternoon following earlier damage to overhead electrical wires.
Commuters were warned on Wednesday morning that all Thameslink services had been cancelled or delayed due to wire damage between St Pancras International and Blackfriars.
Network Rail said shortly after 8am that its engineers had fixed the fault and that trains were moving, but warned there are still cancellations and delays in place.
A Thameslink spokesman said disruption to its services is expected to last until 2pm, but added: “This is subject to change”.
In a Twitter post, Network Rail said: “There were a variety of connected incidents that caused today’s problems.
“It started at around 1am when a train driver reported sparks on the rails. The railway here is powered by the third rail, an electrified rail on the ground.
“This is supported by ceramic insulators known as pots and in this case a pot had cracked and blown.
“This also caused some damage to the signalling equipment. At City Thameslink, the railway moves from 3rd rail power to overhead wires so it also led to damage to the wires.”
Network Rail said trains are now being reintroduced gradually in order to avoid further congestion.