According to a Network Rail (UK) spokesperson, the reason why UK rail can’t withstand 40 degree heat is because it is designed to cope with -10 winter temperatures and a maximum of 35C. Isn’t the temperature range even more extreme in continental Europe?
Yes. We clearly also don’t often get the wrong sort of leaves on the line 
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SNCF have advised that trains running on older tracks should go a little slower, however TGV tracks are more modern and designed for all weathers and so there has been no restriction (so I was reliably informed by the news on F3).
I seem to recall in precious years eurostar dropped to 240kmh, so an extra 15 minutes or so on the journey.
The main problem in France due to the heat is not so much the tracks, but the power lines for the electric trains that have a tendency to break when a pantograph hits them at speed due to thermal sag.
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I have that problem in the winter 
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Wintry weather in France especially on TGV and Eurostar routes suffer from les caténaires freezing up, c’est la vie, c’est exceptionnel! Quand il fait froid, c’est normal. But haven’t heard trains not running because it’s the wrong sort of snow to deal with.
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