4 ms·

It's not so much the materials - modern rail, especially for high speed trains is laid in long continuously welded sections. The track is pre-tensioned so that

It's not so much the materials - modern rail, especially for high speed trains is laid in long continuously welded sections. The track is pre-tensioned so that at cold temperatures as it tries to shrink it is under significant tension and as it warms it becomes less tensioned. The question is what temperature to tension it to - at some temperature it is no longer under tension and above that it becomes in compression and then you have risk of the rails buckling. If I recall correctly, in England the track is tensioned for 27C (a typical hot summers day temperatures). They could tension for a higher temperature, but then you risk rails cracking in very cold weather. So in the end it's a compromise, but one that was made without expecting the current 40C temperatures.

jaclaz 4y ago
The relevant HN thread, a few days ago: https://news.ycombinator.com/item?id=32118818 https://news.ycombinator.com/item?id=32118818
iggldiggl 4y ago
> at some temperature it is no longer under tension and above that it becomes in compression and then you have risk of the rails buckling. If I recall correctly, in England the track is tensioned for 27C (a typical hot summers day temperatures) While the basic gist is correct, a certain amount of compression is fine, too, because the trackbed can resist a certain amount of sideways buckling force, especially once the track has settled in a bit. It's only when the buckling forces caused by the temperature expansion and resultant compression exceed the sideways resistance of the trackbed that you get buckling. Besides, rail temperatures on a hot summer day in the sun will definitively be higher than 27 °C – that value just happens to be the neutral temperature between maximum allowable compression and maximum allowable tension, whatever those temperatures exactly are.
- mhandley 4y ago
  Yes, exactly. And once it's in compression even at higher temperatures it will often be fine if left alone. This is why railway companies impose speed restrictions when it gets hot - they don't want the dynamic load of a passing train to exceed the ability of the track bed to resist buckling. Running at low speeds helps in this respect.
- Someone 4y ago
  https://www.networkrail.co.uk/stories/why-rails-buckle-in-britain/ https://www.networkrail.co.uk/stories/why-rails-buckle-in-br... says “rail can actually be up to 20 degrees higher.” and “For some of our track, such high temperatures are more than our track is designed to cope with”, so it depends on the track, but can be around 50°C. A rail that’s in the shade for most of the day will get less hot, so I expect location also influences at what air temperature track will buckle.

jaclaz 4y ago

The relevant HN thread, a few days ago: https://news.ycombinator.com/item?id=32118818 https://news.ycombinator.com/item?id=32118818

iggldiggl 4y ago

> at some temperature it is no longer under tension and above that it becomes in compression and then you have risk of the rails buckling. If I recall correctly, in England the track is tensioned for 27C (a typical hot summers day temperatures) While the basic gist is correct, a certain amount of compression is fine, too, because the trackbed can resist a certain amount of sideways buckling force, especially once the track has settled in a bit. It's only when the buckling forces caused by the temperature expansion and resultant compression exceed the sideways resistance of the trackbed that you get buckling. Besides, rail temperatures on a hot summer day in the sun will definitively be higher than 27 °C – that value just happens to be the neutral temperature between maximum allowable compression and maximum allowable tension, whatever those temperatures exactly are.

mhandley 4y ago
Yes, exactly. And once it's in compression even at higher temperatures it will often be fine if left alone. This is why railway companies impose speed restrictions when it gets hot - they don't want the dynamic load of a passing train to exceed the ability of the track bed to resist buckling. Running at low speeds helps in this respect.
Someone 4y ago
https://www.networkrail.co.uk/stories/why-rails-buckle-in-britain/ https://www.networkrail.co.uk/stories/why-rails-buckle-in-br... says “rail can actually be up to 20 degrees higher.” and “For some of our track, such high temperatures are more than our track is designed to cope with”, so it depends on the track, but can be around 50°C. A rail that’s in the shade for most of the day will get less hot, so I expect location also influences at what air temperature track will buckle.