Moorgate station has reopened after a "small track fire" closed its national rail station.
A spokesman for the London Fire Brigade said it had sent three fire engines after it was called to a track fire at 10.50.
The Northern, Hammersmith and City, Circle and Metropolitan Lines were running as normal.
However, Great Northern trains were delyed, with the 12.05 service between Welwyn Garden City and Moorgate cancelled.
National Rail said tickets were being accepted by the London Underground.
Only in London: police horses dispatched to fire at Moorgate station. pic.twitter.com/EJj8FRq7CZ— Jamie Sport (@JamieSport) August 18, 2016
A Great Northern spokesman said:
"All lines were blocked between Finsbury Park and Moorgate, but services may be delayed or subject to alteration, but we'll be working to restore the train service to the normal timetable as quickly as possible, although delays may be experienced until early afternoon."