Suspended Railways

Suspended railways are not a common mode of transportation. In Europe, the best-known example is the Wuppertal Suspension Railway, opened in 1901. Two examples exist in Japan, which is more willing to experiment with nonstandard rail technology. With essentially just these three examples in normal urban rail usage, it is hard to make generalizations. But I believe that the technology is underrated, and more cities should be considering using it in lieu of more conventional elevated or underground trains.

The reason why suspended trains are better than conventional ones is simple: centrifugal force. Train cars are not perfectly rigid – they have a suspension system, which tolerates some angle between the bogies and the carbody. Under the influence of centrifugal force, the body leans a few degrees to the outside of each curve:


If the train is moving away from you, and is turning left, then the outside of the curve is to your right; this is where the body leans in the image on the right. This is because centrifugal force pushes everything to the right, including in particular the carbody. This increases the centrifugal force felt by the passengers – the opposite of what a tilt system does. A train is said to have soft suspension if this degree of lean is large, and rigid suspension if it is small. The depicted image is rotated 3 degrees, which turns 1 m/s^2 acceleration in the plane of the tracks into 1.5 m/s^2 felt by the passengers; this is the FRA’s current limit, and is close to the maximum value of emergency deceleration. There are no trains with perfectly rigid suspension, but the most recent Shinkansen trains have active suspension, which provides the equivalent of 1-2 degrees of tilt.

On a straddling train, this works in reverse. A straddling train moving away from you turning left will also suspend to the right:


It’s almost identical, except that now the floor of the train leans toward the inside of the curve, rather than to the outside. So the suspension system reduces the lateral acceleration felt by the passengers, rather than increasing it. By softening the suspension system, it’s possible to provide an arbitrarily large degree of tilt, limited only by the maximum track safety value of lateral acceleration, which is not the limiting factor in urban rail.

This is especially useful in urban rail. Longer-distance railroads can superelevate the tracks, especially high-speed tracks, where trains have to be reliable enough for other reasons that they never have to stop in the middle of a superelevated curve. Some urban rail lines have superelevation as well, but not all do. Urban rail lines with high crowding levels routinely stop the trains in the middle of the track to maintain sufficient spacing to the train ahead; this is familiar to my New York readers as “we are being delayed because of train traffic ahead of us,” but the same routinely happens in Paris on the RER. This makes high superelevation dicey: a stopped train leans to the inside of the curve, which is especially uncomfortable for passengers. High superelevation on urban rail is also limited by the twist, i.e. the rate at which the superelevation increases per linear meter (in contrast, on intercity rail, the limiting factor is jerk, expressed in superelevation per second).

Another reason why reducing curve radius is especially useful in urban rail is right-of-way constraints. It’s harder to build a curve of radius 200 meters in a dense city (permitting 60 km/h with light superelevation) than a curve of radius 3 km outside built-up areas (permitting 250 km/h with TGV superelevation and cant deficiency). Urban rail systems make compromises about right-of-way geometry, and even postwar systems have sharp curves by mainline rail standards; in 1969, the Journal of the London Underground Railway Society listed various European limits, including Stockholm at 200 meters. The oldest lines go well below that – Paris has a single 40-meter curve, and New York has several. Anything that permits urban rail to thread between buildings (if above ground), building foundations (if underground), and other lines without sacrificing speed is good; avoiding curves that impose 30 km/h speed limits is important for rapid transit in the long run.

Suspended railways are monorails, so they run elevated. This is not inherent to the technology. Monorails and other unconventional rail technologies can go underground. The reason they don’t is that a major selling point for monorails is that their sleek structures are less visually obtrusive when elevated. But underground they can still use the same technology – if anything, the difficulty of doing emergency evacuation on an elevated suspended monorail is mitigated on an underground line, where passengers can hop to the floor of the tunnel and walk.

I’d normally say something about construction costs. Unfortunately, the technology I am plugging has three lines in regular urban operation, opened in 1901, 1970, and 1988. The 1988 line, the Chiba Monorail, seems to have cost somewhat more per km than other contemporary elevated lines in Japan, but I don’t want to generalize from a single line. Underground there should not be a cost difference. And ultimately, cost may well be lower, since, at the same design speed, suspended monorails can round tighter curves than both conventional railroads and straddle monorails.

Despite its rarity, the technology holds promise in the most constrained urban environments. When they built their next new metro lines, disconnected from the older network, cities like New York, London, Paris, and Tokyo should consider using suspended railroads instead of conventional subways.


  1. Bjorn

    I never thought I’d see you pushing a novelty mode.

    At the risk of veering into fantasyland, would you view suspended railways as viable for greenfield construction in mountain areas? (I’m thinking of the I-70 corridor in Colorado)

    • Alon Levy

      I think suspended railways are less useful in mountainous areas. Explanations:

      1. Greenfield railways can be built with high cant and reasonable cant deficiency, limiting the extra benefit coming from suspended railways. For the same reason, trains on greenfield railways don’t tilt, unless you count active suspension.

      2. While running a suspended monorail at 80 km/h in an urban area is not a novelty, running one at 160 km/h is. Electrification alone would be a challenge, though most likely a surmountable one.

      3. Even in the mountains, it’s useful to run the above-ground parts of an intercity railroad at-grade. You lose that ability with suspended railways. In urban areas you’re not running anything at-grade so it’s not as big of a deal.

      4. Outside built-up areas, the emergency evacuation problem is a bigger deal. In areas inaccessible except by railroad, what do you do when passengers need to evacuate the train?

  2. snogglethorpe

    If the ability of a suspended monorail to “swing free” and reduce the impact of high speeds on passengers is one of its strong points… wouldn’t that mean larger tunnel diameters would be required for underground systems, to give the trains space to swing…?

    You don’t want the train scraping the tunnel walls at high speed!

    • Alon Levy

      Maybe? Trains are typically taller than they’re wide, so if they swing, even if they swing 13 degrees (the maximum capability of Pendolino technology), the effect on diameter isn’t large.

      • Colin Parker

        This is probably right for a single track in it’s own tunnel. For two tracks (forget the tunnel even), you might need more space between them on curves, but probably not too much more. Maybe you have to worry about platforms too, because the ceiling to floor distance is fixed for a suspended train but the floor to suspension distance in a conventional design could be much less.

        OTOH, if you’re not up against true lateral force limits, why not build each car as a rigid frame with suspension from the top? Would this be too much unsuspemded weight?

        • Alon Levy

          I think the suspension-from-the-top is how the Talgo tilt system works? But it requires specialized bogies and frames, and the current technology only works with unpowered bogies, so you can’t have EMUs. It may also require the train to be very light because of unsuspended weight, but it’s always better to be light for other reasons (energy isn’t free).

          • Max Wyss

            Note: it may get a bit off-topic here; moderator, feel free to delete this entry if it goes too far.

            The suspension in the Talgo acts from a point above the center of gravity of the vehicle, making it tilting.

            There is another system, developed in the late 1980s by SIG for the Swiss Federal Railways, which works with “normal” bogies, driven or not. Roughly said, with that system, the carbody sits on a trapezoid frame with four joints, and the longer side at the bottom. With this, the rotating axis of the carbody can be brought above the center of gravity, without a high suspension point, and in theory, the carbody compensates centrifugal forces. This principle was called “Neiko”, Neigungskompensator, tilt compensatior. It worked reasonably well in prototypes, but did not get developed further, because the SBB changed their doctrine to active tilting (where SIG developed an electro-mechanical solution (as opposed to Fiat’s electro-hydraulic one), which is working well in the ICN trains, as well as the British Pendolino trains).

            After the patents ran out, Bombardier took those concepts and developed it to the “Wako”, Wank-Kompensator, which is supposed to be used in the new TWINDEXX Swiss-Express bi-level EMUs for SBB. These trains are awaiting certification, but may get into regular operation soon, and then we will see whether WAKO will actually be used. The main reason for WAKO is not gaining time (with some 3°, you can’t go thaaat much faster through curves), but to prevent the trains to lean outside of the loading gauge, requiring redoing many curves.

          • Alon Levy

            Wait, why can you not go faster through curves with 3 degrees? It’s another 78 mm of cant deficiency. Maybe the TWINDEXXes have such a high center of gravity (they’re bilevels) that normal cant deficiency is already close to their safe track limit? Because evidently Pendolini manage 300 mm of cant deficiency without derailing, and they’re light but not all that light.

          • Max Wyss

            Actually I was not quite precise in my statement. WAKO will not be used to get additional tilt; its aim is to keep the carbody parallel to the track’s cant, therefore compensating the centrifugal forces (and also compensating leaning into the curve when stopping in a superelevated curve. This type of bi-levels uses the dynamic envelope of the gauge to the maximum, and any extension of that space would cost a lot.

            It has nothing to do with safety; that would require significantly higher centrifugal forces. Maximum axle load of a tilting train (is 17 t, whereas the TWINDEXX SwissExpress may get to 20 t, especially when fully loaded.

  3. adirondacker12800

    …there is a reason why almost no one has rushed out to build one. Probably many reasons.

  4. Brendan Dawe

    Suppose it’s Vancouver, 1982. The BC Government have brought you in and are asking for your opinion on wherther they should go for a proposed suspension railway, the ICTS system they’re pitching out of Ontario, or a conventional LRT between Waterfront Station and New Westminster.

    What do you recommend?

    • Alon Levy

      …I don’t actually know. In 1982, there’s one fewer example of a working suspended railway (and, to be fair, no working example of Bombardier’s ICTS). There’s also a nice straight ROW, so for the most part the advantage of suspended railways on curves is eliminated. Either option is far superior to light rail – are they purposely trying to get the train stuck in Downtown traffic, or what?

      • BDawe

        I suspect any post-1980 light rail plan is going to make use of the Dunsmuir Tunnel downtown, so it will at least be grade-separated at that point.

          • Brendan Dawe

            Have you thought of any real-world lines that would have been better as suspension railway?

          • Alon Levy

            The Yurikamome and Nippori-Toneri Liner would both have greatly benefited from being suspension railways. They have really narrow curve radius, and being able to take it at higher speed would be good for them. The Oedo Line might have benefited, too – it would allow sharper curves to thread between older tunnels, and maybe also steeper gradients (but its linear motor technology has a fairly high grade-climbing capability as well).

            In the Western world, it’s possible Paris M14 might have been easier to build as a suspended railway, and the same is true of Berlin’s U55.

          • Sascha Claus

            Berlin’s U55 is planned to be extended eastwards and connected to the U5 at Alexanderplatz, as was the original plan for this route. So we have an example of the one problem inherent in all ‘novelty’ modes: incompatibility with existing systems (if existing).

          • Alon Levy

            Is it planned to be a U5 branch, or just a line connecting to U5, the way M3bis and M7bis connect to M3 and M7?

          • EJ

            No it’s an integral part of the U5. They just ran out of funding for the bit between Brandenburger Tor and Alexanderplatz.

          • Sascha Claus

            “The way M3bis and M7bis connect to M3 and M7” started out as both stubs being part of the main M3 and M7, respectively. They only became M3bis and M7bis when the main lines were extended, starting from a few stations before the terminus, and thereby making the former terminals to stubs.

  5. Untangled

    The big selling point of suspended trains here is that they are more comfortable in urban situations because it can “tilt” more. Honestly, my response to that is to tell passengers to suck it up instead of going to a novelty technology. Urban railways are generally more comfortable than a bus anyway. I don’t see it as an alternative to full metros anyway, might as well suck up the extra cost if you’re going to build something like that, it’s really only a good alternative to light rail or light metros.

    As for superelevated urban railways being uncomfortable because it stops all the time, I think that stopping on a cant is still more comfortable than going around curves that are not on cants, so just build urban railways superelevated anyway is my thought. Of course, I realise that not everyone will like this, I’m quite used to it though because Sydney does this, even at stations. Now, this is not entirely acceptable at stations but I’m used it. Example, notice the train in relation to the fence.

    • Alon Levy

      Cant excess and cant deficiency are equivalent, yes, even though a lot of regulatory regimes wrongly disagree. But the specific case of a train stopped on the track may be different. There’s no motion, just an inward force, and this is uncomfortable. It also, by definition, lasts longer than going quickly through a curve at cant excess.

  6. Untangled

    There’s no motion, just an inward force, and this is uncomfortable.

    It is uncomfortable but I don’t think it’s that bad just based on my own experience, maybe I just got used to it.

    True that going through a curve is probably over quicker than stopped but if a stopped train on a cant has no forward motion and only a sideways force, I feel that this force comes in only gradually as the train decelerates. A train in motion going quickly around a non-canted curve also has sideways force but I feel that this force sometimes comes much more abruptly, almost like a jolt, which is more uncomfortable imo but it is over much faster as you say.

  7. FDW

    So this is what Seattle should’ve done instead of the bus tunnel and Link? Interesting…

  8. Eric

    Wouldn’t construction costs be much higher, since you need a doorframe-shape to hang the vehicle from, rather than just a flat surface to rest it on? And most materials are stronger in compression than tension? I think these extra costs would eat up the gains from sharper curves.

    • Alon Levy

      That’s possible. The construction costs of the Chiba Monorail were not thaaaat high, but were higher than other els in Japan.

      Underground, though, there shouldn’t be a difference.

  9. Martin

    My biggest annoyance with elevated rail of various kinds (in particular in Taipei), is that the stations infrastructure are usually very poor. Because the stations, and each additional exit costs, are ugly, expensive, and imposing, there always seems to be a shortage of exits in elevated systems (a pet-peeve in many cities that seem to under-supply rail exits). The same issues also seems to result in that stations are built for very short trains limiting capacity (once again Taipei being a good point). The New York elevated stations are also notorious for poor elevator/escalator access (which would require additional imposing infrastructure).

    The only cities where I could see it work pretty well, are places like HK where a large part all of inner city urban life in any case takes place on elevated walkways (but then of course the elevated rail would collide with the current pedestrian infrastructure).

    Usually I think cities are better of with draconian grade separation at street level (you have to accept that it will cut of most road crossings, and maybe build some overpasses/tunnels for passing traffic), or just underground rail. Street level access saves passengers lots of time, and is just better urban planning. Better to spend the money on (real) grade separation. Just how ugly and loud elevated railways are, is of course a major reason for such priorities.

    • Alon Levy

      I don’t think accessibility is really the issue. Yes, New York has poor wheelchair accessibility, but that’s true underground, at-grade, and above-ground. Conversely, Singapore has systemwide step-free access, including on the long elevated tails of its first two MRT lines and its automated people movers (“LRT”).

  10. Jay Yudof

    It also strikes me that monorail, whether suspended or straddle, has a less obscuring superstructure. This may make it more tenable in places where traditional elevated track railway would be rejected by public opinion.


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  11. Michael James

    Two examples exist in Japan, which is more willing to experiment with nonstandard rail technology.

    Seems that those were created in France (pic link below) based on the SAFEGE system, if constructed by Mitsubishi; and the two Siemens suspended monorails in Germany (Dortmund University & Dussledorf airport) are based on the French system too.

    Siemens have discontinued the system but (a bit like another rail system that shall go nameless!) the Chinese appear to be taking it up (in Shanghai & Wenzhou). They mostly seem novelty applications unless part of their rationale is saving space on the ground–eg. putting down the median strip of road without impacting road space (the Dortmund system seems a bit like that)? Or getting a “free” use out of an existing bridge structure–like the Memphis Mud Island monorail which is suspended under a high cross-river bridge (where bridges are expensive due to the scale of the river and the significant commercial river traffic).

    It might also be that for these specialty applications, aerial tramways/gondolas are filling the niche, with obvious advantages. Like the recent London, Paris and La Paz examples.

    • Ken Lee (@Koverptw)

      Look into the private developer-built and developer-run Skyrail system in Midorizaka, Hiroshima connecting to Seno station. A hybrid between suspended monorail and cable car ropeway / gondola lift / aerial tramway (fuck those terms and definitions) for the sole purpose of real estate property hillside.

  12. fyziks

    > The depicted image is rotated 3 degrees, which turns 1 m/s^2 acceleration in the plane of the tracks into 1.5 m/s^2 felt by the passengers

    How did you get this? I might imagine you calculating it as a vector of length sqrt(2) projected onto axes perpendicular and parallel to the floor, then taking the ratio of horizontal and vertical forces. This gives cos(42)/cos(48) = 1.11 m/s^2.

    • fyziks

      Ok I was erroneously using 1 instead of 9.8 for gravity, sorry! Feel free to delete my comment.

    • Alon Levy

      A degree of tilt adds about pi/180 ~ 0.017 g, which is about 0.17 m/s^2. I’m doing it in the ghetto sin x ~ x way, not with actual calculation of angles or anything.

      • EJ

        Alon, please don’t use “ghetto” as an adjective. I know you’re not American, but that’s something young people in America do when they want say something racist but it’s socially inappropriate.

        • snogglethorpe

          Alon, please don’t use “ghetto” as an adjective. I know you’re not American, but that’s something young people in America do when they want say something racist but it’s socially inappropriate.

          No it’s not. It’s widely used slang typically meaning something like “comically jury-rigged / low-quality”.

          • Alon Levy

            By association of low-quality stuff with black people, no? In Israel, which is less PC than the US, the expression for that is “Arab work” or “Arab labor.”

          • snogglethorpe

            By association of low-quality stuff with black people, no?

            Or poor people with few resources.

            You could certainly argue that the term is inappropriate for such reasons.

            I’m just saying that its intended meaning in modern culture is fairly neutral, and most people who use it aren’t doing so because “they want say something racist.”

          • adirondacker12800

            There is a lot of widely used slang that isn’t used in polite conversation.

  13. Ken Lee (@Koverptw)

    Monorail, suspended or straddle-beam, still suffer from the same physical limitations: capacity and track switch. Flexibility and room for future expansion is limited, aside the general issue of identifying potential corridors to reserve space or infrastructure as a provision. As an independent system it serves its niche: mountainous/hillside terrain (light) metro substituent or people mover. Better used in a single line shuttling back and forth than as the backbone in any major network. If one needs to squeeze in any sort of transport within serious space constraint (avenue/highway median, densly-built urban areas) by all means do it than to have nothing transit. Tokyo reflects the beauty of conventional rail metro in its ability to integrate with overhead line dc suburban rail. It and NYC, London exemplifies diversified service in conventional metro for a complex network.
    Would rather look into linear motor in steel or rubber rail systems.

    • Alon Levy

      Does it require far less disruption? The rail still needs to be put in the road. The sitework is the same – it doesn’t scale in the number of rails, which is why single-tracking is so rare even on streetcars that could in single-track with how low their frequency is.

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