Hot weather speed restriction, affecting rail travellers on the East Coast Main Line who use GNER trains, have been lifted.
Restriction affecting GNER trains have been lifted
But travellers on the busy West Coast route are still facing big delays because of restrictions brought in because of soaring temperatures.
Many train companies are running services at a slower than usual speed, amid concern tracks may buckle in temperatures of 30C plus.
Network Rail imposed a limit of 60mph on tracks where trains would normally travel at 110mph.
The West Coast mainline and the Cross Country network are among the hardest hit, with some long distance services halved to one an hour.
But the restriction on GNER were lifted on Tuesday afternoon.
The head of the Strategic Rail Authority, Richard Bowker, blamed Netwrk Rail's predicessor Railtrack for some of the delays.
He said the former privatised company had allowed areas of track to deteriorate.
The worst-hit UK station has been Birmingham New Street, where half the trains to London were cancelled and Virgin passengers faced average delays of 45 to 60 minutes.
First Great Western said 17 services between London Paddington, South Wales and the West Country are being cancelled on Tuesday because of the speed restrictions.
The heatwave has prompted calls from animal welfare campaigners, who say farmers should not send their livestock to market in the current temperatures.
Compassion in World Farming told the BBC's Farming Today programme that animals are at risk of suffering or dying as many British lorries do not have air conditioning and markets tend to be packed.
But livestock auctioneers say there is plenty of space for animals as August is a quiet month for trade.
And the Road Hauliers Association said animals are moved in the early morning or a night when temperatures fall.
At London Zoo, ice lollies made from fish were fed to penguins, while tigers and lions had lollies filled with
one of their favourite herbs - rosemary - while sloth bears and monkeys were
given fruit lollies containing pieces of apple, pear and orange.
And at Newquay Zoo, potbelly and middle white pigs were covered in factor 25
suntan lotion to prevent them from burning.
On a visit to Glasgow, even the Prince Of Wales was moved to joke with factory workers that the weather was "bloody hot".
Forecasters say temperature records could be broken this week.
The hottest day recorded in the UK - 37.1C (99F), recorded on 3 August 1990 at Cheltenham, Gloucestershire - could 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."
London was the hottest place in Britain on Monday at 32C (90F) - even
hotter than Barbados (31C/88F), where the prime minister and his family are currently on holiday.
The Department for the Environment, Food and Rural Affairs warned the
heatwave conditions were causing "summer smog", which could lead to health
High levels of ozone were expected today in London, southern and central
England and south Wales.
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