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Comment by A321321 | Hacker News Reader

Yes, there is. High speed rail quite often uses ballastless track [0]

[0] https://en.m.wikipedia.org/wiki/Ballastless_track

To add on, a lot of underground mass rapid transit uses ballastless track, too.

And street-level light rail, for the obvious reason of sharing the street (even if not lanes) with other vehicles, as well as aesthetics.

TL;DR "ballastless track" is track rigidly set in concrete instead of the aforementioned floating, but it must be done to very tight tolerances and is expensive

If the tracks are rigidly set in concrete, how do they deal with thermal expansion then? Roads made from concrete usually have gaps every couple of meters, but you can't have that many gaps in railroad tracks I suppose.

Either way they buckle: https://www.networkrail.co.uk/stories/why-rails-buckle-in-br...

https://www.youtube.com/watch?v=_LoXgN1QWZM

https://www.youtube.com/watch?v=dqq0543JWzI (wiggle wiggle)

First of all, in the images shown in that article the rail clips are rigidly set in concrete, and the rails are clipped in place. With enough force they can slide longitudinally in the clips.

Second, in most modern rail installations the track is pre-tensioned to deal with thermal expansion. The track is stretched with a hydraulic tool and welded while under tension, so thermal expansion just means reduced tension. This is why trains on modern rail lines don't make the familiar click-clack sound.

(How much tension is applied depends on the country and weather - you can choose between tolerating high temperatures and tolerating low temperatures. This is why in the UK, there are sometimes train stoppages in very hot weather)

>trains on modern rail lines don't make the familiar click-clack sound.

What does the click-clack sound have to do with thermal expansion? Is it because the track is expanding as the train moves over it, if so what’s causing that sound signature?

It comes from the wheels passing over the joints in the track segments. Track that isn't continuously welded is made of individual pieces of rail joined at the ends with a fishplate:

https://en.wikipedia.org/wiki/Fishplate

The expansion gaps.

Theyre not set “in” concrete. The rails are still connected to the sleeper/tie via a fixing plate/spring/shoe, allowing horizontal movement between them. There are still gaps with the concrete bed for that thermal expansion as well.

No. I think your understanding is faulty. The rails are hundreds of meters long: the longitudinal expansion would still have to be accounted for somewhere.

The rails are firmly fixed, longitudinally as well as laterally, with pretensioning in order to minimise stress as they heat up. This is true with ties as much as with a concrete bed.

https://interfacejournal.com/archives/644

"There are a number of ways to increase the resistance of rail to buckling. The first and most common is the use of rail anchors to limit the longitudinal expansion of the rail. Anchors come in a variety of designs, but they all apply several thousand pounds of clamping force on the base of the rail. When applied properly against each side of the crossties, the anchors prevent expansion of the rail in the longitudinal direction."

The general approach is described here: https://en.wikipedia.org/wiki/Rail_stressing

Interesting! One argument I see against hyperloop is the need for expensive vacuum bellows to deal with thermal expansion. I wonder if there is a locked in place solution like this that could eliminate the need for them.

I don't think you could just pre-tension rails so much and be done with it somehow, considering... temperature of a rail can probably stretch between -10 to +50C, and coefficient of expansion for steel is 12*10^-6/K, let delta T = 60K and joined rail length = 1km, the expansion can be +/-36cm(~1.2ft). At some point, the error can be more than one rail segment worth, and maybe it's just me but I can't imagine that amount of expansion just solved by clever tensioning alone.

Usually they are joined to a rail with a slant cut or a tongue section at ends, and let ride into the opposing cuts/tongues, IIUC.

The pre-tensioning isn't what prevents expansion.

The rail can only expand if it can move. If the rail is clamped down tightly to the sleepers, and the sleepers are tightly packed in the ballast, then it just doesn't expand. The only thing that happens is that stress increases / decreases as temperature changes.

The pre-tensioning is only to ensure that the average tension in rail in the course of an average year is in a certain range.

wtf.

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