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These are unused rails, left over from a collapse of civilization on another world.

Assumptions:

  • the ties are impervious. In this regard, their science is greater than ours.
  • the rails run through a mountain pass that is indistinguishable from the US Rocky Mountains.
  • they have a solution for dealing with obstructions like plants and landslides
  • the cart trying to use the rails is a hand cart with two passengers and what they can carry. They're going to build the cart, so they'd have little trouble moving it over/around damaged spots.

How many centuries would it take for the surface to become too pitted from corrosion to make it a viable transportation option?

Update, based on questions: The plan is to build a handcart and use it as a method of crossing plains (like Kansas) and mountains (like Colorado) faster than they would do so on foot. The travelers are incredibly mechanically inclined, and building a handcart from available materials is a trivial task.

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    $\begingroup$ "Thanks to advancements in steel manufacturing, the quality of steel used for railroad track has gotten much better. Today, the lifespan of rail averages 50-60 years, depending on how much freight is transported annually on the line and other factors." You're obviously not using modern safety standards when you say "too pitted from corrosion." It sounds like you don't even want a handcar to run on it. Can you be more specific about your limits? $\endgroup$
    – JBH
    Oct 29, 2023 at 7:33
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    $\begingroup$ I know you said they're indestructible, but it really does feel like realistically, the ties would be the first thing to go. Even if they don't break, they would grow weak enough that they can't keep the rails from trying to push apart. $\endgroup$
    – Cadence
    Oct 29, 2023 at 8:41
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    $\begingroup$ A while ago, i tried to see if you can make "Snowpiercer" work. You can, for like a short time. The issue you face with the tracks is that only a few small pieces have to fail for the track to be useless. By having the track run through mountain terrain, movements in the Earths crust will make it defunct long before a decade has passed. Let alone the constant danger or rock slides, snow and other factors. If you want to make a track last, it has to run through geologically and meteorologically constant areas. $\endgroup$
    – ErikHall
    Oct 29, 2023 at 9:39
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    $\begingroup$ @JBH in railroads there is a term that translates to 'rust riding'. Trains driving over the beams removes rust, allowing signalling where the train is via the tracks, but also maintaining the tracks. For the (rudimentary) signalling it is recommended to drive at least once a day over the tracks, or you might lose it thanks to rust. Besides driving over it, regular inspection and maintenance is still required. A single warped beam is enough to derail a train. On a long enough track and different seasons it could break much earlier, long before the stated 'pitted from corrosion' takes place. $\endgroup$
    – Trioxidane
    Oct 29, 2023 at 22:22
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    $\begingroup$ If you want the answer in centuries, I think its "0 to 1". There may be a more interesting answer in terms of decades or years. $\endgroup$
    – Atog
    Oct 29, 2023 at 22:48

2 Answers 2

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Railway track is intended to last around 100 years according to this study, however that is for heavy locomotives and rolling stock travelling at their full design speeds. Some rails may degrade and be replaced in as little as 30 years with corrosion and heavy use.

However, we're talking about a handcar here, not a heavy locomotive. Since we're talking about a light, slow vehicle that applies very little force to the rails, rails such as "e" in the following image could reasonably be expected to be useable by a handcar This may be as much as a couple of hundred years old. It would be a rough ride, and it might be necessary to lift it over particularly bad sections of track, but depending upon metallurgy, the rails may be two or even three hundred years old and still be useable... for that value of useable.

Don't even think about putting a full-sized train on it though. It will derail.

enter image description here

EDIT:

Since the OP wants a rate of corrosion per year, this abstract shows that carbon steels (from which railway rails are usually made) in water may corrode at around 10-100 μm/year, averaging 50 μm/year. In a 'rocky mountains-like' area, we wouldn't expect great amounts of salt which tends to accelerate corrosion.

If we consider the profile of a typical railway rail:

enter image description here (Image from https://en.wikipedia.org/wiki/Rail_profile)

Corrosion tends to happen where water beads on the metal surface. Water on these rails would be falling on the head and foot of the rail, and would tend to run more quickly down the web of the rail. As can be seen from the photos of corroded rails, it is the underside of the foot of the rail where the most corrosion occurs, mostly where the rail contacts the sleepers. The head of the rail would be most exposed to sun and wind, which would tend to dry the rail more quickly after rain.

As I have previously said, while a rail may be corroded to the point that running a heavy train on it would lead to derailments, this would probably occur on a bend where the centrifugal forces push outwards against the rails and cause failure of the corroded rail foot. On straight sections of track, rather large failures of the entire rail are required to cause derailments, as can be seen in https://en.wikipedia.org/wiki/Rail_sabotage, particularly the video https://en.wikipedia.org/wiki/File:Derailment_226-b-6082.webm, which shows the considerable lengths that saboteurs must go to to successfully derail a train on a straight section of track.

Since the OP has stated that a handcar will be used, we can expect that even an extremely corroded rail will be able to support such a light vehicle for hundreds of years past the point at which a heavy train would inevitably derail.

In Are stainless steel railway tracks worth it?, I asked if stainless steel rails would be practical, as these might last unused in the weather for thousands of years or more, but the consensus appears to be that for a working railway where track wear is an issue, stainless steel is too expensive and doesn't last long enough, and regular high-carbon steels will probably be used. However, only the OP can say if these tracks are intended to be low-use, low-maintenance stainless tracks, or high-use, regular-maintenance high-carbon-steel tracks.

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  • $\begingroup$ Assuming the rail sleepers would last that long, or else deterioration of them would cause the rails to slightly increase spacing, making those handcar runners have to shift wheels or carry it over several miles of changed spacing. With concrete sleepers, this is unlikely since there is no tectonics or overgrowth declared, but wooden ones could decay. $\endgroup$
    – Vesper
    Oct 30, 2023 at 8:20
  • $\begingroup$ Most of the online sources state that rail lifespans are estimated to be around 30 years. In the paper you link, it actually also reads "The design lifetime of railways is sometimes stated to be 100 years (life-cycle), but the lifetime of a certain rail track is generally no more than 30 years based on proper inspection, maintenance and replacement." $\endgroup$
    – Joachim
    Oct 30, 2023 at 9:34
  • $\begingroup$ @Joachim Quite true. Some of the corroded rails in the image might be as little as 30 years old... or they may be considerably older. However, we're talking about the usability of rails by a handcar, which would be much less sensitive to rail quality. $\endgroup$
    – Monty Wild
    Oct 30, 2023 at 9:55
  • $\begingroup$ Decent, well-researched answer, but it presumes that the rails are in use. That information is readily available. I'm looking for something like "millimeters of corrosion per year for railway alloys" $\endgroup$ Oct 30, 2023 at 18:44
  • $\begingroup$ @RobertRapplean Iron-based alloys (that aren't stainless) don't just corrode, they swell up, they corrode unevenly, so you can't simply say that you lose x millimetres per year all around. It's about useability for a given purpose, and while a rail may last 30 years for heavy rail, for a handcar, it'll be much longer. It depends on the environment and the specific alloy... it's another world, so maybe they used stainless steel after all, then you're really talking about millennia. $\endgroup$
    – Monty Wild
    Oct 31, 2023 at 0:18
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Frame Challenge: As long as you need them to

With the phrases "another world" and "their technology [was] greater than ours," you shouldn't be worried about the longevity of the rails. If you are hand-waving the ties (and trestles and spikes and clips and foundation), hand-wave the rails. Assume they are coated with a corrosion-impervious meta-material or something, or leave it a mystery that baffles your characters.

Frame Challenge II: Be careful of too much capability

The travelers are incredibly mechanically inclined, and building a handcart from available materials is a trivial task

If this is the case, building a human-powered land vehicle that can travel over rough-ish terrain should prove similarly trivial. Make a bicycle-wagon with big wheels and enough gear reduction to traverse any reasonably walkable landscape. If this doesn't fit your theme, all the more reason to handwave the rails

Compromise by waving only half a hand

Edit based on comments. You could say that these advanced vehicles were more capable versions of modern Maglev trains. The guide rails, therefore, could be mechanically strong steel electroplated with a thick layer of pure gold (which would be very common in that world). Gold doesn't react with typical atmospheric gasses. That removes wear and friction as maintenance concerns, and removes the need for a complicated sci-fi alloy. The service life of these rails would only be limited by their supporting infrastructure.

The electrical mechanisms essential to levitation have long since ceased functioning of course, but the gold-plated railway is more than strong enough to support a custom-built handcar.

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  • $\begingroup$ w.r.t Frame Challenge II: a railway alignment, even with all the track equipment rendered unusable, is still a long, flat, shallow-sloping road between urban centres. $\endgroup$
    – dplane
    Nov 4, 2023 at 15:44
  • $\begingroup$ Re Frame Challenge 1: While I accept that I can make them last as long as I need them to, I'm looking for a baseline above which such extremes would be necessary. If you hand-wave too much, then you are no longer writing science fiction. If I were to ask about ties, trestles, spikes, clips, and foundation, then the question would be flushed as "too many questions in one." $\endgroup$ Nov 5, 2023 at 1:17
  • $\begingroup$ Re Frame Challenge 2: Yes, the guy could (and does) make walking vehicles that would make terrain a non-issue, but they would take much longer to build, wouldn't be as durable as a simple rail car, and would be more likely to be stolen before his return trip. Also, they're less fun. $\endgroup$ Nov 5, 2023 at 1:22
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    $\begingroup$ @RobertRapplean Understood, just had to throw my two cents in there :) My point in FC2 was a small caution against plot holes. Obviously you want a train-track solution. I'll update my answer with another idea $\endgroup$
    – automaton
    Nov 8, 2023 at 18:29
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    $\begingroup$ Ooo, love the mag-lev track concept. That would work, and bonus points for reuse of ancient technology in a way that wasn't intended. $\endgroup$ Nov 8, 2023 at 20:39

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