COMMUTERS have been warned to expect delays tomorrow due to heat-related speed restrictions on the rails. 

Rush hour travellers faced woes this evening as services up and down the country, including Southern Rail and Thameslink, had to run their trains at reduced speeds. 

On warm days rails in direct sunshine can be as much as 20 degrees hotter than air temperature - causing them to expand.

With the weight of trains compressing this expanding rail can put the track at risk of buckling.

If the track buckles the line must be closed and rails must be repaired - usually work which cannot be completed until the pieces cool. 

A Network Rail spokeswoman warned all train passengers to expect delays during rush hour, and again tomorrow as the mercury is set to rise. 

She said: "There are speed restrictions all over the country due to the hot weather so there will be some delays and some disruption. 

"We apologise to passengers for delays but obviously the restrictions are in place to minimise disruption on the network."

The heatwave is being caused by a warm front and tropical continental air mass from Europe pushing across the country, bringing high temperatures, humidity and possibly Saharan sand.

Tomorrow is expected to be even hotter, with temperatures predicted to hit 35C (95F), making a level 3 heatwave alert likely.

The highest recorded July temperature was at Wisley, Surrey, in 2006, where the mercury peaked at 36.5C (98F).

Mick Cash, general secretary of the Rail, Maritime and Transport union, said: "It's bad enough travelling on Britain's rammed-out, overcrowded trains at the best of times, let alone when temperatures are heading to boiling point and our members are on full alert to assist passengers who are in distress.

"It is only right that some serious leeway is given to workers in the expected heatwave conditions and once again this situation reinforces the importance of having train and platform staff on hand and trained to deal with any eventuality."