Trains have been delayed after speed limits were put in place across the UK as temperatures edged towards record levels.
Holidaymakers were out in force in Scarborough on Monday
Network Rail imposed the limits on most of Britain's busiest lines, amid fears of rails buckling in temperatures of up to 33C.
It meant journey delays of up to an hour for commuters trying to get home on Monday afternoon and early evening.
The speed restrictions were lifted at 1900 BST as temperatures cooled, but were expected to be imposed again on Tuesday and in the coming days.
The limit brought trains which normally travel at up to 110 miles an hour, down to 60.
The West Coast mainline and the Cross Country network were among the hardest
hit by the restrictions, with some of the long distance services halved to just
one an hour.
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The worst-hit station was Birmingham New Street, where half the trains to London were cancelled and Virgin passengers facing average delays of 45 to 60 minutes.
"The train was an hour late and they're now telling us to get off the train because it's too crowded," one annoyed passenger said.
Steve Hounsham of Transport 2000 questioned why the rail network could not cope with the temperature extremes of "what is, after all, a relatively mild climate".
Other analysts queried why the speed limits were put on entire lines, rather than just potentially vulnerable areas of weaker track.
UK'S HOTTEST DAY
UK's hottest recorded temperature is 37.1C (99F)
Recorded on 3 August 1990 in Cheltenham, Gloucestershire
'Could be beaten this week', say forecasters
A former British Rail director blamed the blanket nature of the speed limits on the splintered nature of post-privatised railways.
"One expects some speed restrictions in weather like this, we've always had problems with very high weather," said Peter Rayner.
"[But] the blanket speed restrictions across the country are of course a consequence of the fact that Network Rail does not have the local knowledge that its predecessor but one did have."
Too hot to shop
The delays came as forecasters said a string of temperature records could be broken this week.
The highest temperature so far this year - 33.6C, recorded on 15 July at Wisley, Surrey - is likely to be beaten mid-week, said the BBC Weather Centre.
And the hottest day ever recorded in the UK - 37.1C (99F), recorded on 3 August 1990 at Cheltenham, Gloucestershire - could also topple on Wednesday or Thursday.
BBC forecaster Nina Ridge said:
"We're looking at temperatures around 10 degrees above average for this time of year."
Even at night temperatures will remain about 17 or 18C across much of England, which could be uncomfortably sultry for some people, she said.
London recruitment agencies told the BBC they had had one of their busiest days, and suspected many workers were taking the day off to enjoy the sunshine.
But some traders expressed concern that people are too hot to shop.
Peter Avey, who runs Seasiders amusement arcade and restaurant in Brighton, said: "Over the weekend numbers of
people were up but takings did not reflect it.
"Everyone was buying ice creams and drinks, but I think it's too hot to
As well as high temperatures this week, there will also be high humidity, causing discomfort or even heat stress for many people.
England will be the hottest area, but Northern Ireland and Scotland will also be in the mid to high 20s.
Forecasters advise vulnerable people - especially the elderly, the very young and those with high blood pressure - to keep cool by staying in the shade, drinking lots of water, and not doing anything too strenuous.
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