Commuters have faced delays and overcrowding as temperatures on the UK’s hottest day caused chaos on the rail network.
Routes across the country, including those serving London’s main railway stations, have been badly affected.
Problems with overhead wires were blamed for many delays, with Network Rail saying the “majority” were caused by the heat.
It apologised to passengers facing “uncomfortable conditions”.
The Rail Delivery Group, which represents train operators and Network Rail, said disruption could last until the weekend.
Director Robert Nisbet said: “Many of the trains will be in the wrong places, many of the rail staff will be in the wrong places and it will take time to reset, to stabilise the timetable again.”
Some commuters reported being stranded inside “baking” carriages as trains came to a sudden halt mid-journey.
Passenger Vicky Planet said her train ground to a halt shortly after leaving Euston.
She said: “From 13:30 we’ve been baking in an oven.”
When she spoke to the BBC, Ms Planet said she was waiting to be transferred onto a train taking passengers back to Euston.
She said bottles of water had been handed out by staff on board, who were “doing all they can” despite also suffering from the heat.
Vivienne Tsui, 23, who was also stuck on a train leaving Euston, said the carriage felt “like a sauna”, adding that that one woman collapsed with breathing problems.
Frank Mee was stuck on an East Midlands Trains (EMT) service from Derby to St Pancras inside a tunnel for “hours” from about 14:00.
He said there was no air conditioning or water and passengers were given no information by train staff about what happened or how long the train would be stationary.
“I am shocked and disheartened,” he said. “I paid £100 for this ticket and they didn’t even give us water – absolutely ridiculous.”
Mr Mee, 24, from Vauxhall in south London, added: “They tried to turn the air con on but there was such a strong petrol smell as a result, they turned if off very soon afterwards.”
EMT has advised passengers not to travel.
A spokesperson said: “We are aware one of our services has stopped near Belsize Tunnel, between West Hampstead Thameslink and Kentish Town. This is due to damage to the overhead line equipment.
“At present, we are not able to run any services south of Bedford.
“The air conditioning and other onboard systems are working, nonetheless, we understand the conditions on the train will be uncomfortable.”
By 16:40, passengers were being taken off the train, according to the EMT spokesperson, who added: “We expect disruption to continue for the rest of the day and urge customers who have not yet set off not to travel.
“Tickets for today will be valid for travel tomorrow.”
Passengers were also stranded on two trains in Cambridgeshire, leading to an evacuation of the train.
London North Eastern Railway (LNER) said its trains had been hit by problems with the “infrastructure in the Peterborough area”.
Overhead power line damage has also affected those travelling in the West Midlands.
Why do overhead wires break in the heat?
Network Rail said overhead lines which provide power to the trains can expand and sag in hot weather.
To avoid damaging the lines, trains must travel more slowly, but if they are damaged, “we have to cancel or divert train services until they are fixed”.
Modern overhead lines, it added, are much less affected by hot weather because they have auto-tension systems with balance weights or springs that adjust to different temperatures.
But older overhead lines have fixed tension and “are more vulnerable”.
Network Rail said it was replacing old overhead lines with modern, more resilient auto-tension power lines, adding that modern lines were used on the “majority of Britain’s rail network”.
Meanwhile, a grass fire broke out on a railway bank near Finchley Road, causing 400 people to be evacuated from a train and 200 from a bus.
A spokeswoman for the London Fire Brigade said the blaze was caused by a damaged overhead line which, in turn, caused a short circuit.
Firefighters were called at to the scene at about 14:40.
The UK has had its hottest July day on record with temperatures reaching 38.1C in Cambridge.
Network Rail announced that speed restrictions would be in place in the south east until 8pm amid fears that tracks could buckle in the heat if trains travel too fast.
Speed limits on most commuter lines will be cut from 60mph to 30mph, it said.
Extreme weather action teams have been “activated” to keep passengers safe and trains running, Network Rail added.
Nick King, network services director at Network Rail, said: “Our teams are working flat out to fix the issues as quickly as possible and get people on the move.”
Network Rail has advised passengers to check before they travel.