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That was more insightful than the whole article. Another thing I was curious about but not answered from the article is if there are any alternatives.
by weinzierl 3y ago
That was more insightful than the whole article. Another thing I was curious about but not answered from the article is if there are any alternatives.
- A321321 3y agoYes, there is. High speed rail quite often uses ballastless track [0] [0] https://en.m.wikipedia.org/wiki/Ballastless_track https://en.m.wikipedia.org/wiki/Ballastless_track
- fsckboy 3y agoTL;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
- weinzierl 3y agoIf 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.
- bombcar 3y agoEither way they buckle: https://www.networkrail.co.uk/stories/why-rails-buckle-in-britain/ https://www.networkrail.co.uk/stories/why-rails-buckle-in-br... https://www.youtube.com/watch?v=_LoXgN1QWZM https://www.youtube.com/watch?v=_LoXgN1QWZM https://www.youtube.com/watch?v=dqq0543JWzI https://www.youtube.com/watch?v=dqq0543JWzI (wiggle wiggle)
- michaelt 3y agoFirst 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)
- atentaten 3y ago>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?
- mauvehaus 3y agoIt 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 https://en.wikipedia.org/wiki/Fishplate
- Ensorceled 3y agoThe expansion gaps.
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- donavanm 3y agoTheyre 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.
- tififtivfj 3y agoNo. 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 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 https://en.wikipedia.org/wiki/Rail_stressing
- jhallenworld 3y agoInteresting! 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.
- numpad0 3y agoI 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.
- delta_p_delta_x 3y agoTo add on, a lot of underground mass rapid transit uses ballastless track, too.
- Sharlin 3y agoAnd street-level light rail, for the obvious reason of sharing the street (even if not lanes) with other vehicles, as well as aesthetics.
- moooo99 3y ago> if there are any alternatives. There are. There are ballastless tracks that utilize a continuous concrete slab instead of the crushed rock ballast. Their main benefit is that they are lower mamintenance than their counterparts, but their upfront cost tends to be higher and construction takes longer, making them difficult to retrofit onto existing stretches (would require long closures). However, the low maintenance nature of this style makes it popular at locations where maintenance is very difficult (long tunnels, etc) or where loads are particularly high, either due lots of use or high speeds, like with new HSR construction.
- jojobas 3y agoHigh speed rail track also don't use expansion joints, the rails are continuously welded, which means that instead of expanding/contracting with seasons, they pack quite some tension or compression, which is basically worn by the strength and friction of these concrete slabs.
- CPLX 3y agoThey do use expansion joints they’re just a little more modern now. Look up “breather switch” for examples. Another common technique is to install the track already heated so it can’t get even larger and buckle.
- jojobas 3y agoThey only use them where needed for other purposes (i.e. electric isolation). They don't require joints for thermal purposes.
- chiph 3y agoThey do (sometimes) preheat continuous welded rail during installation to prevent buckling. The temperature used will vary based on the local environment (Minnesota will use a lower destressing temperature than Florida). Shorter sections of track (like in a switching yard) may not be destressed at all because there isn't enough length to cause a buckle. > Compressive forces result from stresses induced in a constrained rail by temperature above its "stress free" state, and from mechanical sources such as braking, rolling friction and wheel flanging on curves. The temperature of the rail at the "stress-free" state is known as the rail neutral temperature (i.e. the temperature at which the rail experiences zero longitudinal force). Initially, the rail's installation temperature or "anchoring temperature" is the rail's neutral temperature. Hence, at rail temperatures above the neutral, compressive forces are generated, and at temperatures below the neutral, tensile forces are developed. Track maintenance practices address the high thermal load problem by anchoring the rail at (neutral) temperature of 95 -110 F. This high neutral temperature range prevents the generation of excessively high buckling forces even when the rail temperatures reach 130 -150 F. https://www.volpe.dot.gov/infrastructure-systems-and-technology/structures-and-dynamics/track-buckling-research https://www.volpe.dot.gov/infrastructure-systems-and-technol... Some railroads that experience wide summer vs. winter temperature differences may choose to install expansion joints. Like most things, there are trade-offs, like increased maintenance costs and increased chance of derailment at the joint.
- matthewmacleod 3y agoThe alternative is generally some form of ballastless or slab track – https://en.m.wikipedia.org/wiki/Ballastless_track https://en.m.wikipedia.org/wiki/Ballastless_track - essentially rail fixed directly to a concrete substrate using a variety of different approaches. Lower maintainance costs at the expense of higher installation costs, and most often seen on high-speed systems, tunnels etc. There are also various forms of baulk track – https://en.m.wikipedia.org/wiki/Baulk_road https://en.m.wikipedia.org/wiki/Baulk_road - in which the rails are supported longitudinally with regular tie-bars to maintain gauge. I don’t think this is used much outside of some specialist applications.