Why Railroads Don’t Need Expansion Joints

Why Railroads Don’t Need Expansion Joints

A friendly overview of thermal effects on railways.
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Errata: At 9:00, the left column of calculations is incorrectly labeled “SI.” It should be imperial. Whoops!

Just as all materials have a mostly linear relationship between temperature change and length change, all materials also have a similar relationship between stress and change in length (often called strain). And this is part of the secret to continuous welded rail: restrained thermal expansion. You can overcome one with the other.

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41 Responses

  1. @InvestmentJoy says:

    Awesome! I’ve seen thermite rail welding before and was curious behind the why’s of it.

  2. @gregyjoebatin2558 says:

    As a civil engineer, this type of content is therapy

  3. @Sam-bn9ix says:

    Railway engineer here! A few points:
    Not all rail is made of the same alloys, particularly high load areas such as points and crossings. These use harder metals to reduce wear, such as manganese. Adding this into the equation means welding plain line to p&c is a particularly complex job.
    I am also glad you mentioned track circuit will be covered later on as that is a big downside to CWR with older signalling systems.
    Aluminothermic welding is spectacular when it goes wrong, search it up!

    • @whuzzzup says:

      So let me ask a question what I wondered about often. So the pictures of buckling rail is because those rails were not installed properly? For example laid in cold winter without heating or applying pre-tension? And/or were the ties in the past worse in keeping the rail straight?
      Is it common for rail to be heated/tensioned in mild climates when a new track is built or does this only happen in more extreme climates like Texas?

    • @Sam-bn9ix says:

      @@whuzzzup if installing CWR most of it needs to be installed and stretched to the length at the stress free temperature rail length. I.e in the UK all tensioned rail is stretched to it’s equivalent length at around 27 Celsius.
      It’s also important to note that this is the rail temperature not air temperature so it may not be very warm but if it is a sunny day the rail will become a lot hotter than air temp. This is when buckling occurs as the rail becomes very hot and has nowhere to move. Majority of this happens when a vehicle is running over the rail.
      As engineers know this we implement critical rail temperatures to ensure that beyond a certain temperature we know the rail is expanding we can gradually implement speed restrictions etc. to reduce these risks to public.

    • @thomaswalters4365 says:

      I can see diagonal tapers being used in between blocks to isolate block signals from each other.

    • @iainstenhouse8399 says:

      @@thomaswalters4365Those are expansion joints. I am as yet to see one used in that application.

    • @salerio61 says:

      Hi Sam, I was a railway engineer too back in the days of BR, worked out of Derby Research. I missed the part where he mentioned track circuits, but I am very interested to see the American slant on it.

  4. @MikeV8652 says:

    If anyone wonders why the odd figure of 39 feet was the standard length of single sticks of traditional jointed rail, it was so that they’d fit into 40-foot gondola cars. Welded rail is now typically manufactured in long lengths (1,320 feet in the USA, which is ¼ mile) and can be welded in the field to be continuous. Such long rails are transported on special rail trains, like the one Grady shows half-loaded at 0:25. The yellow racks hold them in place, and the rails bend as the train goes through horizontal and vertical curves 😮.

    • @royreynolds108 says:

      Actually the modern rails are rolled in about 30-meter lengths and then welded into the quarter-mile lengths to be transported to the laying site and then welded by a portable electric welding machine or thermite welding process in track.

    • @MikeV8652 says:

      @@royreynolds108 The fabricating of quarter-mile lengths is part of the manufacturing process. I intentionally avoided saying that the rails were rolled that long.

    • @ferky123 says:

      The rails are only “held in place” in the middle. They lay on the rest of the structures.

    • @MikeV8652 says:

      @@ferky123 True that they’re fastened down only at the middle, but the racks surround them to keep them aboard and untangled, to guide them to bend around curves, etc.

    • @thomaswalters4365 says:

      Didn’t know that about the 39′.
      Makes a lot of sense.
      Had to transport the rail to the site somehow back then. Gons were 40 foot.

  5. @crispybassist says:

    As a engineer who works in railway track design, one thing I’d like to point out – especially since you mentioned a video on signaling will be coming soon – is that neutral temperature is typically high not because a broken rail is less dangerous than a sun kink. Yes, its more likely that a train could operate over a pull-apart rather than theough a sun kink, but largely because the signal system will detect a pull-apart as a discontinuity in the rails and thus the signal circuit – automatically turning the signal ahead of that track to red!

    • @mikefochtman7164 says:

      I’ve always been amazed at how railroads developed signalling systems. And how the idea of ‘fail-safe’ design was involved waaaayyyy back in the early 20th century. Broken rails, loss of power, relay failures were all considered and will set signalling ‘safe’.

    • @TSBoncompte says:

      ooooooooooooh, right, cause if you use disjointed rail you have to have a separate cable for the signal to travel on, at least in every joint. so even if pointy joins solved the expansion issue the added wires and cases and little welds sounds like a lot of extra work.

    • @Tupsuu says:

      Im interested to know how that works. Im pretty sure I saw bare copper used as grounding for the rails last time I paid attention to it. Or have I missed something

    • @crispybassist says:

      @@Tupsuu It depends a lot of the specific railway line, that could have been traction power return cables, grounding, equalizers between adjacent tracks, signal infrastructure, etc…

    • @Torchedini says:

      @@mikefochtman7164 That fail safe system was paid for in blood though. But since rail is that old most of the accidents aren’t that highly publicized or live in recent memory like development of the airplane and space flight.
      It is kind of interesting to see how safety culture around cars is so low, but that is I think a question of high amount of owner operators/low barrier to entry. On rail most things are owned by large corps, which has similarities to aerospace as well.

  6. @pschlik says:

    Fun fact: The problem with ‘rock n’ roll’ (more formally, harmonic rocking) is bad enough that rulebooks disallow travelling at certain speeds on jointed rail. The rulebook I have says this danger zone is 13 to 21 mph, so if your train only has enough power to reach 20 mph (entirely possible, if normal, these days) safety says you’d have to slow to 13 mph on jointed rail. So CWR not only makes maintenance-of-way’s job easier, it also makes operating the trains a bit easier and safer too.

    • @erich1394 says:

      r e s o n a n c e

    • @royreynolds108 says:

      @@erich1394 I think you both are correct. As a civil engineer who spent over 20 years in track design and work, the term “harmonic rocking” is the one used by the railroads with certain speeds over jointed track with alternating low joints. It is the alternating low joints that cause the problem not the joints by themselves. There was a lot of research done to determine this phenomina.

    • @miinyoo says:

      So I think your point is; stay out of the resonance zone or transient through it as quickly as possible. Go slower or faster so you don’t build up a resonance that can, not always but can go exponential. Blown mind all these little details that add up.

    • @Trump2Prison says:

      @@royreynolds108 Also bogey (or track) hunting. I forget which car type, one had a both a lower and upper speed limit when unloaded due to excessive gauge resonance that lead to $many derailments.

  7. @petepi says:

    “Stress is what breaks things” is a truth that more people should apply to their own lives

  8. @pyrokaren says:

    I developed a broken rail detection system at San Francisco BART that used ultrasonic waves traveling miles in the continuously welded rail. As part of characterizing the rail I measured acoustic properties as a function of rail temperature. It was amazing to see the rail temperature exceed 140 degrees F on medium hot days, like air temperature in the 90’s. The peak temperature was in the late afternoon when the rail web caught most of the sunlight. Definitely painful to touch!

  9. @xavierburval4128 says:

    I love that the primary solution for buckling is just “do it on a really hot day lol.” It’s the kind of thing that sounds way too dumb to work but is somehow the most genius solution ever.

  10. @PapaDalbec says:

    My man, promoting math to the masses. We need more folks like you!

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