Thousands of rail passengers faced huge disruption as delays hit three major London transport hubs on the hottest July day on record.
Commuters were stuck at St Pancras railway station after overhead cables were damaged by the heat, severely disrupting East Midlands and Thameslink services.
Wires between Euston and Watford Junction were also damaged in the scorching temperatures, disrupting Virgin Trains services.
Meanwhile a trespasser at Waterloo - reportedly a man who chased after his dog on the tracks - caused thousands of South Western Railway commuters to be left stranded.
Photos posted on social media show huge crowds gathered inside and outside the stations on Thursday evening.
Euston was temporarily closed due to overcrowding as services ground to a halt.
Boards displayed at the stations announced trains had been cancelled and others were delayed.
Jake Neill, who was visiting London from near Glasgow, told the Standard: “I've never seen anything like it.
“This was the busiest I had seen Euston ever within all the times I’ve used it.
“People were getting flustered and were bothered as the trains were getting cancelled and delayed.
“It was very difficult to move around and it was sweltering inside with the heat despite the stations air con being on full blast, was a relief to get outside.”
It came as the UK saw its hottest July day and second hottest day on record as the mercury hit 38.1C.
East Midlands Trains earlier posted a message on Twitter urging passengers "DO NOT TRAVEL" and warned it had been unable to secure ticket acceptance via alternative routes.
Thameslink said "you are strongly advised not to travel" and said journey times will be extended by up to 90 minutes.
A spokeswoman for Virgin Trains said: "Due to extensive disruption on the network today, any Virgin Trains customers who would prefer to postpone their travel can use their tickets on Virgin Trains services tomorrow.
"A full refund will be available to those who choose not to travel."
Nick King, network services director at Network Rail, said: "We have a number of heat-related incidents across the rail network this evening that are causing disruption to services.
"We are sorry that some passengers are experiencing uncomfortable conditions and inconvenience.
"Our teams are working flat out to fix the issues as quickly as possible and get people on the move.
"We're asking anyone travelling this evening to check with their train operators or visit the National Rail Enquiries website to see how their journey is affected."
