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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 Jul 5 '17 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 Jul 5 '17 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 Jul 5 '17 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 Jul 5 '17 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. Jul 5 '17 at 11:11
  • $\begingroup$ @einpoklum: "(example - 30 metric tons per car)" 14 tons per car for monorail. $\endgroup$ – SF. Jul 5 '17 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. Jul 6 '17 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. Jul 6 '17 at 17:16

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