Commuters were yesterday stuck at St Pancras railway station after overhead cables were damaged, severely disrupting East Midlands and Thameslink services.
Boards displayed at the station announced most trains had been cancelled with others delayed.
Heatwave temperatures had already pushed the timetables to breaking point yesterday.
A reduced timetable in the South East came into force at midday yesterday as Network Rail implemented speed restrictions amid fears tracks could buckle in the heat if trains travel too fast.
There has been disruption on the London St Pancras/Nottingham/Sheffield line which continues today due to damage to overhead electric wires.
East Midlands Trains said that Network Rail are on site trying to repair the extensive damage.
They added that, whilst this work is ongoing, a reduced London service will be operated on Friday 26th July on an emergency timetable.
They have also advised commuters not to travel on the line.
A spokesman said: "There is damage to the overhead electric wires near Belsize tunnel. This is between London St Pancras and Luton and is disrupting our trains on the London St Pancras / Nottingham / Sheffield routes.
"Network Rail are on site trying to repair the extensive damage. Whilst this work is ongoing, a reduced London service will be operated on Friday 26th July on an emergency timetable.
"There is currently no estimate for when all lines between London St Pancras and Luton will reopen and a full service can be operated. Any customers that have essential travel are advised to check live journey planners before they travel.
"We are sorry for the disruption caused to your journey."
A Network Rail statement said Friday tickets would also be accepted tomorrow, adding: "East Midlands Trains strongly urge you NOT TO TRAVEL on the London St Pancras International / Nottingham / Sheffield route today."
A full timetable can be found here.
Commuters may travel to Doncaster where they should change for an LNER train to London Kings Cross.
They can also travel to Manchester Piccadilly with TPE and change for a Virgin Trains service to London Euston