from Hacker News

Why do railway tracks have crushed stones alongside them?

by Twixes on 6/4/23, 7:36 AM with 283 comments

• by londons_explore on 6/4/23, 8:16 AM

  This article doesn't explain one interesting bit:

  It is important that these crushed stones stay clean. If dirt gets in them, known as fouling, filling the gaps between the stones, then eventually it will cause a derailment.

  Briefly, the stones move slightly as each train passes above them. The tracks 'float' on the denser stone. This effect helps keep the track straight and level - higher regions get pushed down, while lower regions raise themselves up.

  Fouling stops this effect, and eventually the track will become so uneven that a derailment happens.

  To prevent this, a big part of railway maintenance involves digging out all the stones, washing them to get rid of sand and soil, and putting them back. Usually done every 25 years or so.

• by ofalkaed on 6/4/23, 10:44 AM

  This gets its purpose regarding plants a bit wrong, the only time roots would be any issue is on tracks which have not been used in quite awhile and at that point the tracks will almost certainly have other issues which need to be addressed before they get used again. The metal on metal of the wheels on tracks often causes sparks and these do occasionally cause fires in dried vegetation alongside/on the tracks which the ballast helps to keep in control, which is why the ballast generally extends further from the edges of the tracks than is needed for stability reasons. If you work track maintenance part of your job will be torching any plants growing too close to the tracks especially on grades where wheels often slip and brakes applied which can produce a surprising amount of sparks. They also have cars which spray weed killers alongside the tracks which run the line once or twice a year depending on conditions.

• by rapnie on 6/4/23, 8:11 AM

  In the Netherlands there is an ongoing debate on the use of ballast stones giving off quartz dust that can cause lung disease and cancer.

  https://www.dutchnews.nl/2021/04/prorail-is-jeopardising-wor...

• by kingkongjaffa on 6/4/23, 8:25 AM

  If you run a high speed train over a ballasted track the under carriage aerodynamics suck the ballast up and they can hit the bogeys and axles and cause cracks in the train axle. If this happens in a bad way it can derail the train.

  This abstract has a good summary https://research.birmingham.ac.uk/en/publications/a-full-sca...

  So the question becomes can we replace ballast with some other mechanism where this doesn’t happen?

• by aurizon on 6/4/23, 8:46 AM

  As time goes by the ballast settles and under repeated heavy loads and the underlying land becomes a little uneven. In comes the Tamper, a ballast tamping machine. This is a long heavy machine that runs on the track.Quite strong it lifts a length of track under load a few inches and tamping arms reach down and force fresh ballast under with dynamic level sensors and moves on down the track. Tracks are surveyed with sensors on trains and ballast tamping/washing/etc are scheduled. https://www.youtube.com/watch?v=WqLynLWLd8Y

• by michaelsshaw on 6/4/23, 8:09 AM

  I was riding the train to work about a year ago and randomly googled this exact issue. I was amazed at how much thought actually goes into designing the ballast, as I had always assumed it was just some random fine gravel.

• by Octopuz on 6/4/23, 3:37 PM

  During a boy scouts summer camp, we were on a separated camping area near a railroad track. We created a fireplace and used some stones from the track to build the fire on.

  A pretty big mistake as it turned out. Because the stones were in fact vulcanic, they exploded when heated - so we were bombarded with stone fragments.

• by jhallenworld on 6/4/23, 2:37 PM

  I watched a video just yesterday about wood ties vs. concrete ties. One of the arguments in favor of wooden ties in the US is that they are less brittle and thus more able to handle the frequent derailments we get. The nightmare scenario with concrete ties is that a loose wheel goes unnoticed for miles and cracks every single tie.

  They mentioned a previous test experience with older concrete ties held together with naked angle-irons. A derailment severed all of the irons.

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

• by Twixes on 6/4/23, 7:36 AM

  Fascinating how such a simple solution is so timelessly effective. A "framework" going two centuries strong.

• by rhn_mk1 on 6/4/23, 9:14 AM

  > Without the ballast in place, railway lines would simply not be able to cope with the stress from the weight of the trains passing over them, not to mention the impacts of expansion and contraction.

  How does this relate to tram lines? They don't seem to have much ballast at all, and they are often just inset into asphalt, so there's no draining, and the asphalt presumably limits expansion and contraction?

• by softgrow on 6/4/23, 10:39 AM

  There is a longer article https://www.ejrcf.or.jp/jrtr/jrtr15/pdf/f38_tec.pdf from a series on Railway Technology from the Japan Rail and Technology Review https://www.ejrcf.or.jp/jrtr/technology/index_technology.htm... . It contrasts ballasted track with slab track and purports a 9 year breakeven on the higher construction cost but reduced maintenance cost. It notes that the main Shinkansen from Tokyo to Osaka is ballasted track and I understand it still is. Mind you how you could ever changeover on this super busy line is going to be somewhat difficult.

• by bsaul on 6/4/23, 12:14 PM

  With time i've come to realize any piece of engineering can be a source of inspiration, even to a software dev / architect like me.

  I feel like it took me a lot of time to figure this out, as i've got 20 years of experience. Anyone else ?

• by fzeindl on 6/4/23, 6:40 PM

  I am working for a railroad company at the moment, we learned cool stuff during onboarding.

  Modern train have an electromagnetic emergency brake system that uses a large magnet lowered onto the rail to stop.

  Also the standard braking system using fluids has a clever indirection where it does not activate the brake when pressure rises but when it drops. This is because when a waggon gets loose and the braking line rips and fluids get's out, the waggon should immediately brake as soon as pressure drops.

• by usr1106 on 6/4/23, 9:25 AM

  It's an exaggeration to claim that ballast would have been used for railway tracks for centuries as the article claims. Railways haven't even existed for 2 centuries. Although there are only 2 years left. Still in the 1970s, only 50 years ago, Finland had railways build with weaker material, small round stones.

• by chromanoid on 6/4/23, 9:24 PM

  Relevant machine that is called Gleisschotterbettungsreinigungsmaschine in German. Nice video for children about it (in German, there's probably an inofficial YT video where you can switch on automatic subtitles): https://kinder.wdr.de/tv/die-sendung-mit-der-maus/av/video-s...

• by dark-star on 6/4/23, 8:10 AM

  Modern high-speed tracks don't use ballast stones anymore though

• by mberning on 6/4/23, 3:15 PM

  There are a lot of parallels with modern hardscape construction. Nowadays many builders grade the soil base, add a woven stabilization fabric, then add clean angular stone, and finish with a finer angular stone on top. I wonder if railroads are using any geotextile fabric under their stone these days.

• by guy98238710 on 6/4/23, 9:29 PM

  The stones must be good reflectors of noise, which makes the ballast rather undesirable under trams in cities. I wonder why is it still used in such circumstances instead of having noise-absorbing soil and grass under and around the tracks as is sometimes done.

• by euroderf on 6/4/23, 4:35 PM

  Several comments mention track buckling in extreme heat.

  When this happens, how is it resolved ? Does the track usually revert to its original positioning at all points, or does every stretch of recently-buckled track have to be inspected and possibly realigned ?

• by cratermoon on 6/4/23, 4:03 PM

  A nice video with good visualizations of the mechanics of the ballest from ABC Science: https://www.youtube.com/watch?v=TlSOMfDX-yY

• by irrational on 6/4/23, 4:46 PM

  My first thought, on reading the title, is maybe the sparks sometimes thrown off by the trains would catch dry weeds on fire if they were near the tracks, so the crushed stones give the sparks somewhere safe to land.

• by wg0 on 6/4/23, 11:29 PM

  Does anyone know about that device on electric engines that touches the wires above?

  Why is that it doesn't spark and do the overhead cables need frequent replacement because of being constantly rubbed against?

• by ericjang on 6/4/23, 7:24 PM

  I entered the title question of the article into ChatGPT, as I was curious whether it could factually synthesize information on the Internet similar to how this article did. Notably, it missed the detail about keeping vegetation away

  <chatgpt> Railway tracks have crushed stones, commonly known as ballast, alongside them for several important reasons. Here are the main reasons for using ballast on railway tracks:

  Stability and Load Distribution: Ballast provides stability to the railway tracks and distributes the load of the trains evenly. The crushed stones interlock with each other and form a stable base for the tracks. This helps prevent the tracks from shifting or sinking under the weight of the trains.

  Drainage: Ballast allows for effective drainage of water from the tracks. The open spaces between the crushed stones allow water to flow away from the tracks, preventing the accumulation of water. Proper drainage is essential to maintain the stability of the tracks and prevent damage caused by water-related issues like erosion or track bed saturation.

  Track Alignment and Adjustment: Ballast allows for easy alignment and adjustment of the tracks. The crushed stones can be added or removed as needed to maintain the proper track alignment and ensure a smooth ride for trains. This flexibility is especially important during track maintenance or when new tracks are being laid.

  Noise and Vibration Damping: Ballast helps in reducing noise and vibrations generated by passing trains. The layer of crushed stones absorbs and dampens the vibrations, minimizing the impact on the surrounding environment and nearby structures.

  Track Maintenance: Ballast provides a protective layer for the tracks. It acts as a barrier between the track infrastructure and the underlying soil, preventing soil movement or erosion that could damage the tracks. Additionally, ballast makes it easier to detect and repair any issues with the track, such as loose fastenings or track bed settlement.

  Overall, the use of ballast alongside railway tracks is crucial for maintaining the stability, alignment, drainage, and longevity of the tracks, ensuring safe and efficient train operations. </chatgpt>

• by andix on 6/4/23, 9:15 PM

  Aren’t roads built in a similar way, just with a different surface?

• by chrisstanchak on 6/4/23, 4:55 PM

  tl;dr

  Crushed stones, or ballast, along railway tracks provide stability, aid drainage, control vegetation, and distribute train load.

• by mintaka5 on 6/4/23, 6:32 PM

  drainage so the rails don't float off somewhere else due to erosion. not alongside them either. under them.

• by x3874 on 6/4/23, 8:42 AM

  ...so many words and just at the end they comment on 'flooding' - whereas everyone knows that a top function of ballast is to prevent water buildup / facilitate drainage.

• by swayvil on 6/4/23, 3:38 PM

  It's purely aesthetic. The sharp edged stone contrasts nicely with the polished rail and the dark woody tie.