Passengers have been helped from a train that was forced to stop when overhead cables snapped and started a trackside fire near West Hampstead.
The fire broke out near Finchley Road at around 4pm. 400 passengers on the London bound Thameslink service were guided across tracks onto another train which was partially the station.
Passengers stuck on trains reported on Twitter that air con had shut down.
On Friday afternoon there were no services between Luton and St Pancras, with disruption expected until at least 10pm.
Thameslink are advising passengers to check their website before travelling.