Comment by tififtivfj | Hacker News Reader

Comment by tififtivfj | Hacker News Reader

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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

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.

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.

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