No trains can run between Bedford and London St Pancras while Network Rail engineers attempt to fix the problem.
East Midlands Railway and Thameslink are attempting to source road transport to get people to their destinations.
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But a spokesman warned passengers stuck on trains not to attempt to get off, adding: "If you are on a train that is unable to move, please do not leave the train.
"Staff are working to safely evacuate a number of trains, and once information is available they will be able to direct you further."
EMR also reported a tree had caught fire close to the railway near Luton.
A tweet from the company said: "Rail replacement buses are being arranged between Northampton and Wellingborough/Kettering/Corby.
"If you are travelling to/from Wellingborough/Kettering/Corby you can use services from London Euston to Northampton and then replacement buses when they arrive."