There have been no trains from Dartford to London Victoria via Bexleyheath this morning (Friday, Jan 27) because of emergency engineering work at Nunhead.
Southeastern has warned commuters that services are unlikely to return before 8.30am.
The 7.10am Dartford to London Blackfriars service via Sidcup was also cancelled. Other trains via Lewisham are running as timetabled.
Southeastern said: "Urgent repairs to the railway between Lewisham and Nunhead means train services running through these stations will be cancelled. Disruption is expected until 8.30am."
Network Rail engineers were trying to replace a broken rail since 5am.
A spokesman said: "We've traced the fault to a damaged insulated block joint which was causing signals to show red. It needed an urgent repair so we had to close the line to complete the work.
"Our team rolled into action around 5am and installed a new rail and insulated rail joint so we could safely run trains."
The damaged rail was between Lewisham and Nunhead.
Network Rail announced the line had been reopened at 8am but warned: "There will likely still be some delays while Southeastern Railway get the service running on time again."
Passengers can use their tickets on London Buses between Lewisham and Denmark Hill and Thameslink services via Nunhead and London Underground between London Victoria and London Cannon Street, Southwark, London Bridge and Elephant & Castle at no extra charge to complete their journeys.
If you are delayed, the train company says you should retain your ticket in case you are entitled to compensation.