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My (highly limited) engineering intuition says: Dual railing is good - for balance, for switching of tracks, for lightening load and making sure it is applied in basically the same direction always, and perhaps other reasons I haven't thought of.

So, why ever have monorails? I mean, ok, if you want to fit into some super-narrow crevice then maybe I can see the motivation, but other than that - what are the benefits?

Note: I'm interested in non-magnetic monorails, although if you want to provide an answer for maglev's as an interesting side-note that's also fine.

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  • $\begingroup$ A monorail system typically still has bearing points (wheels or similar) on both sides of the support rail. $\endgroup$
    – Carl Witthoft
    Commented Jul 5, 2017 at 15:32
  • $\begingroup$ @CarlWitthoft: Yeah, but they're close together. Otherwise it's really a dual rail with the two rails being connected by solid material... $\endgroup$
    – einpoklum
    Commented Jul 5, 2017 at 16:47
  • $\begingroup$ Two exist : one in Japan and the other in Germany in a place called Lathen (although it may have been implemented by now). Lathen was the test centre and I had the pleasure of a full tour and a ride : 0 to 440kmh back to 0, very very smooth bends and all. check out maglev trains. $\endgroup$
    – Solar Mike
    Commented Jul 5, 2017 at 21:58

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One of the factors is reduction of amount of infrastructure per kilometer (= cost).

Monorail is usually elevated on pylons, which means little disruption to the land below (which, in case of normal railway is immense). A single narrow line will exert less torque on the pylons than a broad dual rail would. Monorail is strictly for passenger transport, so there's no need for broad rails that would distribute the load over a broad area, as with cargo transport. The shape of the train allows better silencing (the train is own sound screen).

All these factors add up to reduction of mass, width, extra infrastructure elements, land usage, and as final result - cost. The trains are more expensive to build, but with railway that's usually a small fraction of cost - the ground infrastructure is the lion share. Monorail minimizes that.

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  • $\begingroup$ Really? 1. Having 15m-20m high pylons, capable of supporting large cars and safe for high speeds, along your entire track, sounds like much more infrastructure/km to me. 2. There's still some disruption below - the massive pylon need quite a bit of support; and you're trading ground area of disruption for total volume. 3. Passenger transport still needs wide, heavy cars whose loads you want to distribute (example - 30 metric tons per car) ... can you give/link to a more concrete example of savings in terms of infrastructure? $\endgroup$
    – einpoklum
    Commented Jul 5, 2017 at 10:33
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    $\begingroup$ Versus a 20 meters high embankment good 5 meters high, with another 5 meters of ditches on both sides, internally reinforced against getting squeezed out, then several drainage layers, thick gravel, communication and power cables in the ground, really heavyweight viaducts over even smallest streams, sound screens near settlements, power infrastructure, embankments often much higher (and broader), and so on. Or just pick the price of a hectare of land and see how long a piece of 2-track railway takes it up. Not in US. Europe, Japan, etc. $\endgroup$
    – SF.
    Commented Jul 5, 2017 at 11:11
  • $\begingroup$ @einpoklum: "(example - 30 metric tons per car)" 14 tons per car for monorail. $\endgroup$
    – SF.
    Commented Jul 5, 2017 at 11:22
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    $\begingroup$ @einpoklum: Yes. And the reply was to the comment right above mine. The "rail above" monorails are usually a gimmick, for sightseeing and such - there's little benefit for such a layout (some benefits for city, where liberties with pylon layout must be taken, plus better views). With standard monorail, there's not only no need for more weight, it can weigh much less, because it doesn't need extra-heavy undercarriage that brings the center of mass lower. (normal rail cars could be easily much lighter if not for risk of derailing.) $\endgroup$
    – SF.
    Commented Jul 6, 2017 at 14:49
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    $\begingroup$ ...and as result, cost of meter of track can be lower. Even ignoring cost of terrain. Consider space taken up by monorail vs "Elevated", the metro segments that go over ground, straddling the streets. These things are MASSIVE. As I said, cost of cars is minuscule comparing to cost of track infrastructure, and you hardly ever run enough trains to overcrowd the tracks - so the simple solution is more smaller trains. And faster too - never mind comfort, a train completing the circuit twice in the same time as competitor means 2x the number of people transported. $\endgroup$
    – SF.
    Commented Jul 6, 2017 at 17:16
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I suspect most (if not all) monorails are vanity projects and/or tourist attractions; it'd be interesting to see some evidence that they're cheaper or better anywhere, except for very short routes in hand-picked locations.

The only monorail that I've used regularly is in the German town of Wuppertal. It is visually intrusive and very noisy going round corners (except when it is about to rain!), but the townspeople love it, as it is a major attraction in an otherwise unremarkable industrial town.

A monorail may appear to be safer, as it is relatively inacessible on pylons, but the truth is otherwise, as sadly demonstrated in the 1999 accident on the Wuppertal line; a relatively minor derailment had some very tragic consequences.

Maglev is the same principle, taken to extreme; it brings old-fashioned railways into the 21st century, but there is a lot of technical complexity; the old steel-wheel steel-rail combination is incredibly efficient, and hard to beat. There is a paper examining the relative costs here

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  • $\begingroup$ How does this square with what SF wrote? $\endgroup$
    – einpoklum
    Commented Jan 25, 2022 at 11:29
  • $\begingroup$ It doesn't; I think SF's arguments take a narrow view of the railway-building process, for example consider how complicated monorail points ('switches' in US parlance) are; this tends to limit a monorail to be a simple single-line link, and not a comprehensive transport network. Also "Monorail is strictly for passenger transport" is not a positive attribute, if we are looking to move goods traffic off our roads onto rail. $\endgroup$
    – jayben
    Commented Jan 25, 2022 at 14:33
  • $\begingroup$ But aren't urban rail networks mostly single-line links? With stations requiring transfers between platforms? And do we expect to move goods traffic onto urban railways lines? (Maybe we do, I just haven't noticed that happening) $\endgroup$
    – einpoklum
    Commented Jan 25, 2022 at 15:46
  • $\begingroup$ The original question wasn't limited to urban railways. $\endgroup$
    – jayben
    Commented Jan 25, 2022 at 16:43
  • $\begingroup$ jayben: I was assuming when you discussed multiple points you meant in urban areas. Are there complicated monorail systems with points/switches elsewhere? $\endgroup$
    – einpoklum
    Commented Jan 25, 2022 at 19:54
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In an urban area, the monorail offers a few benefits compared to the traditional trains.

  1. It requires less space overland, thus, it is a better option for the developed cities to provide additional means for transportation.

  2. As monorail runs on the elevated track, it has no traffic crossing, so it causes no traffic congestion, accidents, and time delay.

  3. Unlike the traditional trains, there is no risk of derailing, so again, it is beneficial in avoiding accidents and time delay.

  4. The modern monorail is driven by an electric locomotive that, in itself, produces no pollution.

  5. As there are fewer moving parts in a monorail, it requires less costly maintenance.

  6. A ride on the monorail is both comfortable and rapid, and its look usually is artistically pleasant, so it likely becomes an attraction to the tourist, as well as the local residents who love the 24/7 convenience it provides.

For Cons, this web article details its disadvantages.

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    $\begingroup$ You're answering the wrong question. I'm not asking about elevated monorails vs non-elevated dual-rail trains, but rather about mono- vs dual- rail with other features being basically the same. So, compare against an elevant dual-rail, electric locomotion, same speed, similar design aesthetic etc. $\endgroup$
    – einpoklum
    Commented Jan 25, 2022 at 19:52
  • $\begingroup$ I don't see the limit (non-elevated) stated anywhere in the question. The subject of your question is "Monorails - why even have them?", and in the text, you ask "So, why ever have monorails? I mean, ok, if you want to fit into some super-narrow crevice then maybe I can see the motivation, but other than that - what are the benefits?". BTW, my answer is equally applicable to the underground monorail system. But I guess it is one of your limits too, but was not spelled out. $\endgroup$
    – r13
    Commented Jan 25, 2022 at 21:54

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