Who, What, Why: Why does hot weather cause rail delays?

from the BBC:





As the hottest weather so far this year hits parts of the UK, train passengers are being warned of further train delays caused by buckling rails. Why does this happen, asks Justin Parkinson.

On hot days, steel rails exposed to direct sunshine can become 20C hotter than the air temperature, according to Network Rail. The resulting expansion can cause extreme compression and buckling. When this happens, lines become impassable and close for repairs, which can't usually happen until temperatures drop again.

Some operators have warned of temporary speed restrictions on stretches deemed most at risk, as slower trains exert lower forces on the track, reducing the risk of buckling.

In the past, the UK's main system of coping with extreme heat involved leaving gaps, known as expansion joints, along the line, allowing rails to slide past each other as they expand. But the small breaks in the line meant trains were noisy and gave bumpy rides.

Under the replacement system, to prevent buckling the track is "pre-stressed" or stretched. Only when it gets unusually hot does the metal expand enough to pose a risk of rails pushing together and buckling. ...............(more)

http://www.bbc.com/news/magazine-33342481