Train company GNER has launched an inquiry into how hundreds of passengers became stranded for hours in stifling heat due to a power failure.
Passengers wait at the trackside for a replacement train
Some travellers broke carriage windows to escape temperatures of over 37C on its 1555 BST Newcastle to London Kings Cross service on Thursday evening.
Several trains were halted in what GNER said was a major incident when cables came down near Peterborough, Cambs.
Medical staff and supplies were flown in by helicopter.
The fire service and police arrived at the track to give people on the GNER train emergency treatment.
Rail services on the East Coast Mainline are returning to normal following the problems, though there is still some disruption.
WAGN train services between Hitchin and Peterborough are experiencing delays of up to 30 minutes and there could be short notice alterations to GNER services.
A GNER spokesman said: "We fully appreciate how unpleasant an experience this has been for passengers and apologise to them.
"A full and detailed investigation into what happened is under way and GNER will learn the lessons which emerge from this."
One passenger who was stranded said people began to panic and faint as water and drinks on the full train ran out.
Michael Steed, 70, who was travelling from York, said: "It was dreadful. At first everyone was quite cheerful, but then we were all in a lot of discomfort.
"People were fainting and repeatedly pulling the emergency cord.
"Eventually a woman having a panic attack grabbed the emergency hammer and broke through the window."
He estimated about 300 passengers were stranded on the track just outside Sawtry.
Heather Moger, 38, of Godmanchester, Cambridgeshire, was travelling from Peterborough to Huntingdon on another train, when it stopped at Conington just before 1730 BST.
She said they eventually got off the train after two-and-a-half hours.
She praised the train driver for keeping everybody calm.
"He just kept walking up and down the train and making sure the children and everyone were all right."
A Network Rail spokeswoman said passengers had been taken off four trains stranded after the power failure and another four trains were turned back to Kings Cross.
The stranded trains began to move southwards shortly before 2200 BST after a relief train had driven from Peterborough to collect passengers waiting by the line and carry them to Peterborough station.
GNER posted an urgent notice on its website saying there was no estimate to the extent of delays to trains and apologised for the inconvenience.