Disruption has continued on Wednesday as repair work takes place across the UK’s rail network.
Trains are being cancelled and rail services disrupted after the UK recorded its hottest day on record.
Network Rail said no direct trains are running between London and Scotland due to heatwave damage.
It urged passengers to check before they start their journeys and only to travel if absolutely necessary.
It comes after Tuesday’s record-breaking temperatures above 40C, which caused damage to overhead electric lines and railway tracks.
Repair work is ongoing across the UK’s rail network, and Network Rail said it has launched a new taskforce to investigate how the railway can become more resilient in the wake of the heatwave.
More than two dozen services leaving from London King’s Cross and Euston stations were cancelled or delayed on Wednesday morning.
Services on the East Coast Main Line, which connects London and Edinburgh, were disrupted after a fire on the track near Sandy in Bedfordshire.
The route between King’s Cross and Peterborough is expected to remain closed until at least midday.
Meanwhile, there are no direct trains on the West Coast Main Line between London and Scotland.
Network Rail, which owns and runs most of the train lines in Britain, warned passengers that journeys will take “much longer than normal” and could involve taking rail replacement buses, as work continues to repair the railway.
James Dean, Network Rail’s West Coast South route director, apologised to passengers impacted by the disruption.
He added: “Specialist teams worked all night but have been unable to fix the cables in time for the start of service this morning. We’re doing everything we can to get things back up and running.
“In the meantime, please check National Rail Enquiries before you set off on your journey as delays will continue today.”
‘There are no trains’
Travel expert Simon Calder said that “hundreds” of passengers were facing disruption at King’s Cross station on Wednesday morning.
“There are hundreds [of people] on the concourse looking up at the board which simply says: ‘There are no trains.’ Everything is cancelled,” he told the BBC’s Radio 4 Today Programme.
“The reason people are still here, rather than moving to another station, is that next door at London St Pancras International, the only places you can go to are Paris, Brussels, Amsterdam or Kent, because all the northbound lines from there are closed.”
It follows severe travel disruption on Tuesday, with most journeys scrapped through the Met Office’s extreme heat zone, which covered much of central, northern, and south-east England.
No services ran into or out of King’s Cross all day.
The cancellations were due to the heat affecting the safety of steel railway tracks, which can buckle when too hot.
Railway speed restrictions were brought in across most of England and Wales, with reports of buckled tracks and overheard wire systems failing on Monday.
Trains that did run were mostly limited to speeds of 90mph, down from 100mph or 125mph, while some had to travel as slow as 20mph.