Last month a freight train carrying a cargo of Guinness came off the tracks near Lichfield in the Midlands.
The driver walked away shocked, he was the only person on board and the crash closed the West Coast Main Line.
Investigators would only have had to look up at the sun beating down on the rails to have worked out what went wrong. The track had buckled in the heat.
That is why Network Rail is being so cautious, and why thousands of rail passengers across Britain have been so disrupted.
Last month's freight train could easily have been a high-speed passenger train.
The problem that is making Network Rail sweat is relatively simple to explain - track is made of steel, and it expands in the heat. More modern track is welded together into continuous runs, making it much smoother but more susceptible to warping.
To prevent it happening, the track is 'pre-stressed' or stretched, so that when it gets hot, the metal cannot expand any further. It is designed to cope with temperatures up to 30 degrees centigrade.
Ah, say the armchair engineers, why not stretch it more so it can take more heat? The problem is that the stretching makes it more likely to crack in cold weather - British track can take our coldest winters.
The main problem is that it is currently much hotter than normal, not just in Britain but across the rest of Europe. The temperature of the track can reach 50 degrees.
Network Rail has taken a head office decision, quickly made public, to introduce speed restrictions, where in the past, restrictions have been introduced locally as and when the condition of the track deteriorates.
It has resulted, for example, in a blanket 60 mile an hour limit on the main London to Glasgow, West Coast line. Trains usually reach speeds of 110 miles an hour on that route.
The company says all its decisions are based on a number of factors - the temperature and whether track is in direct sunlight, the type, speed and frequency of the trains and the condition of the track.
In the case of the West Coast Line, Network Rail insists the blanket limits been introduced because there are so many sites where buckling could occur, trains would be constantly slowing down and speeding up, which would be even more disruptive.
But most of all, the increasing vulnerability of our railways to summer speed restrictions is more to do with the poor quality of the track.
Railtrack built up a backlog of 4000 miles of railway lines which should have been replaced but were not.
In particular the gravel used to pack down the rails and sleepers has worn away. This ballast, as its called, stops the track moving from side to side, which often happens when it buckles.
That is why British railway passengers have to put up with delays when in the stifling heat of the south of France, the high speed trains are unaffected.
Track abroad is often just a few decades old and well maintained. Some can even be set up for higher temperatures, then reset once it gets cooler.
Only one line in Britain is of a similar standard - the new high-speed line to the Channel Tunnel, built to French standards - but it will not open until September.