Rail passengers are being warned they face a week of delays because of speed limits imposed in response to soaring UK temperatures.
Many trains ran slower than usual for the second day on Tuesday, amid concern tracks could buckle in temperatures of 30C plus.
The limits were lifted from 1900 BST as temperatures cooled, but Network Rail said they would be imposed again at noon every day until temperatures drop below 30C.
Tuesday was the hottest day of the year so far, the BBC Weather Centre said - with 34.6C (94.3F) being recorded at Jersey Airport. Wednesday is forecast to be even hotter.
Passengers were trapped on a train in sweltering heat in Cumbria for four hours.
The Virgin service from Plymouth to Edinburgh broke down at Tebay, where passengers were initially told doors or windows could not be opened even though the air conditioning had failed.
Nicola Jones, en route to Carlisle, said there was a "greenhouse effect" on the train.
"The windows are huge on new trains and you could not open them. "
Ms Jones said people were panicking and begging for doors to be opened.
The West Coast mainline and the Cross Country network were again among the hardest hit on Tuesday, with some long distance services halved from two every
hour to one.
Journeys on the stretch of line between
London Euston and Birmingham were taking up to 45 minutes longer due to the
speed restrictions, which reduced 110mph trains to as slow as 60mph.
The BBC's Robert Hall in Birmingham New Street station said the atmosphere there was "gloomy and sticky" during the Tuesday evening rush-hour.
"We are saying to passengers do expect some cancellations and to expect some
delays, but services are actually running and people are getting to where they
want to go," Network Rail said.
In other effects the hot weather has been having:
- Animal welfare campaigners said livestock should not be sent to market in the current temperatures, although farmers and hauliers said they had various ways to make animals comfortable
- Store chain Argos sold out
of paddling pools - but introduced its winter range including Christmas trees
On a visit to Glasgow, the Prince Of Wales joked with factory workers that the weather was "bloody hot"
- The AA urged motorists to take care not to lock their children and
pets in cars by mistake, and to take breaks if they were tired from not sleeping properly at night
- Ice cream manufacturers predicted record sales for the year, with hot and dry conditions meaning ice cream "flying out of the door"
- The TUC called on the government to impose a maximum temperature for the workplace, beyond which workers would be allowed to go home
Zoos fed ice lollies made from fish, fruit and herbs to penguins and animals to keep them cool, and put suntan lotion on some to stop them burning
- British Gas urged householders to do the unthinkable and
turn on their central heating, to avoid breakdowns.
Health workers have warned that children and the elderly, especially those with asthma, could be at risk from the heat and should be careful.
The Department of Health has issued a "ten tips" list reminding people how to stay safe in the high temperatures.
The tips 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.
The hot weather has brought a boom to struggling British seaside hoteliers and traders.
"It's 45-50% up on a year ago, the heatwave helped tremendously. We've got a tremendous number of people walking in," said Duncan Moir of the Royal Hotel in Weston-Super-Mare in Somerset.
The hottest weather on the mainland on Tuesday was 33C (91F) at Gatwick Airport.
Wednesday could see temperatures in the south-east nudge the record of 37.1C recorded in Cheltenham in 1990, forecasters said.
The heatwave looks set to continue through the weekend and into next week,
with temperatures again in the mid-30s.
BBC forecaster Nina Ridge said temperatures were about 10 degrees above average for this time of year.
Temperature extremes by continent:
Africa: 57.8C, El Azizia, Libya, 13 Sep 1922
Antarctica: 15.0C, Vanda Station, Scott Coast 5 Jan 1974
Asia: 53.9C, Tirat Tsvi, Israel, 21 June 1942
Australasia: 53.3C, Cloncurry, Queensland 16 Jan 1889
Europe: 50.0C, Seville, Spain, 4 August 1881
North America: 56.7C, Death Valley, California, 10 July 1913
Oceania (Pacific Rim): 42.2C, Tugnegareo, Philippines, 29 April 1912
South America: 48.9C, Rivadavia, Argentina, 11 Dec 1905
Source: BBC Weather Centre