A train was re-routed after its driver spotted a rail line which had been buckled by the high temperatures.
Trains are being forced to go slow because of the hot weather
Network Rail said the incident was a good example of why thousands of passengers are facing delays due to speed restrictions imposed during the heatwave.
The driver spotted the buckled rail at 1530 BST on Tuesday as he waited to leave platform two at Gatwick Airport station.
The stretch of line was closed until Wednesday - the hottest day of the year as temperatures reached 35.5C in Wisley, Surrey.
A Network Rail spokeswoman said: "This clearly demonstrates the problems the heatwave can cause us, and why the current speed restrictions are required.
"On this occasion, an alternative track was used and there was no delay to passengers and we would like to stress that there was no danger to the train."
But people trying to cool down in the hot weather has led to two tragic swimming accidents in England.
Mark Attwood, 17, was found dead in a canal lock in Rotherham, South Yorkshire, after police divers were called to the scene.
The teenager was swimming at Hurstbourne Priors, Hampshire.
And a 17-year-old construction worker, Christopher Jones from Pontefract, West Yorkshire, died in a lake in Hampshire.
He had been helping to build the man-made lake at Hurstbourne Priors, Whitchurch, when he went for a swim but never surfaced. His body was later recovered.
The incidents prompted warnings from police and rescue workers against leaping into lakes and canals as the heatwave brought temperatures of more than 30C across the country.
Staff day off
The heat in London prompted bosses at the London Eye to close the tourist attraction.
A spokeswoman said it was decided to close the popular tourist attraction "for the comfort of guests".
People with tickets could get a refund or rebook. The ride may reopen later on Wednesday if the temperature cools, the spokeswoman added.
Forecasters believe temperatures will not top the record of 37.1C recorded in Cheltenham in 1990.
In Berkshire, police officers had to buy bottles of water from local service stations to hand out to motorists caught up in miles of tailbacks on the A34 after a lorry crash near Newbury at about 1200 BST.
Meanwhile, a firm in Lancashire gave workers the day off on Wednesday.
Staff at advertising firm JGM in Rawtenstall will be making the most of the sunshine - away from their desks.
The heatwave looks set to continue into the weekend.
That could mean many more days of rail delays for commuters.
Network Rail will impose speed restrictions at noon each day until temperatures drop below 30C.
The speed restrictions on the trains brought severe delays to passengers on the West Coast mainline and the Cross Country.
And journeys between
London Euston and Birmingham were also taking longer than normal.
Elsewhere a group of passengers on a Virgin Train were trapped on a sweltering train for about four hours on Tuesday when it broke down in Cumbria.
Virgin Trains said an investigation was under way into the incident which happened because of a "technical fault" with the service.
Other effects the hot weather has been having:
The Department of Health has issued "ten tips" for how people can stay safe in the high temperatures.
These include sitting in the shade or indoors; covering up or using sunscreen; drinking more water and less alcohol; ventilating your home and avoiding physical exertion.
There may be the odd shower in the southeast later on Wednesday, but many parts will have a dry and very hot day with sunny spells but patches of mist will affect some coastal areas.