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After reading this thread from a bit under a year and a half ago about the most cost-effective means of travel on Mars, where one of the answers mentions rail as the best option, I started wondering: since rail travel depends on the friction between the wheels and the rails, which is proportional to the pressure holding the two together, which is proportional to the weight on the wheels, which would be much less on a low-gravity world than on Earth, might this make lighter rail vehicles, such as trams, unworkable or impractical on low-gravity worlds?

Some problems I thought of:

  • The reduced weight on the wheels would weaken the adhesion between them and the track, making them far more prone to wheelslip than Earthbound trams (especially when accelerating quickly from a dead stop or braking hard from [relatively] high speeds, as the low gravity would decrease the weight on the wheels, but not the tram's momentum, as its mass would remain the same). This could be solved or mitigated by:
    1. having wheel bogies that wrapped around the track (like with rollercoaster trains), which would greatly complicate switching (ordinary switches would almost certainly not do the job; transfer tables or something about as complicated would be required, which would take up much more space than an ordinary switch and be more complicated to build, operate, and maintain) and make a dedicated right-of-way an absolute requirement (trying to maintain the required clearance under and to the sides of the track would be an absolute nightmare in street-running situations, and would be a headache even for the very short stretches required for grade crossings);
    2. using a rack-railway system, which would also complicate switching, to a greater or lesser degree (depending on the precise rack system chosen), and might (again, depending on the precise system used) also make street-running difficult or impossible;
    3. using some method of moving the trams not dependent on wheel-on-rail friction, such as cables (would pose difficulties with switching, would make it difficult or impossible to cross moveable bridges or anything else that produced intermittent discontinuities in the track, would be prone to sudden, large-scale disruptions of service due to cable breakages), linear motors (as in many maglev designs, but this method of propulsion does not require that the vehicle being propelled be a maglev; would likely require a completely enclosed ROW to prevent or mitigate the serious issues that would otherwise result from the powerful magnetic fields required, which would make grade crossings flat-out impossible and could cause difficulties with switching), or hyperloop-style air propulsion (would also require a completely-enclosed ROW [although this time because it would be to increase efficiency, requiring far less air to be moved than with an open ROW], creating the same problems as the previous option).
  • The reduced weight on the wheels would also make the trams more prone to derailment, by making it easier for the wheels to ride up and over the rails; this could be reduced or eliminated by using wraparound wheel bogies or completely enclosing the ROW (each of which would have their own problems, as described above), or by making the trams much heavier (which would greatly degrade the already-subpar - see above - starting and stopping distances, as well as considerably increasing energy use by the trams).

So, would low gravity make trams and other (relatively) lightweight rail vehicles unworkable or impractical?

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  • $\begingroup$ While derailment hazard can be addressed, reduced friction will affect all wheeled forms of transportation, not just rail. $\endgroup$ – Alexander Mar 14 '18 at 23:20
  • $\begingroup$ Earlier trains had massive wheels - both in size and weight - to address friction. Maybe a similar solution could be had here? But I have no idea if it would be worthwhile - There's a lot of things that can be done but are impractical to do. $\endgroup$ – Andon Mar 14 '18 at 23:21
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    $\begingroup$ @Andon: I was under the impression that the reason they were so large was because most steam locomotives didn't have gearboxes... $\endgroup$ – Sean Mar 14 '18 at 23:25
  • $\begingroup$ I clearly remember reading about friction when it came to wheel sizes on earlier trains. I'm trying to find it now. $\endgroup$ – Andon Mar 14 '18 at 23:28
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    $\begingroup$ By the way, how low gravity is low gravity? If we're talking Mars I doubt it would be a problem. Mars gravity is 0.38g, so in terms of reducing the friction it's equivalent to the car being about a third as heavy. If this doesn't provide enough friction it could be countered by, for example, reducing the number of wheels, to concentrate the force in a smaller area (which is probably possible since they don't need to support as much weight) or putting grips on them. But if we're talking about the Moon or somewhere smaller than that it could be more of an issue. $\endgroup$ – Nathaniel Mar 15 '18 at 7:34
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Simply solved.

Keep in mind railroads already have this problem, because steel-on-steel adhesion is quite poor, with only about 40% adhesion even possible, with sand application.

Track brakes

Track brakes are attached to the bogey (truck) and contain electromagments. They are positioned above the railhead, and fill most of the distance between the two axles. They are tied to the suspension so they can ride quite close to the rail. Track brakes are able to be run by batteries.

enter image description here *

Now there are two modes to operate a track brake. First, line it with brake pads and give it free motion downward to clamp onto and slide along the rail. This is the normal operation method today.

Second, "pin" it so it does not have free motion and cannot contact the rail. Now it is magnetically pulling the wheels down upon the rail, increasing adhesion on the wheels. I'm not aware of this mode being in practical use today.

Bigger flanges

Look at tram flanges -- they are very short compared to railroad flanges, because of the way street railway rail is built. (also a tram derailing is not a big deal). Use railroad flanges on your trams. That was easy.

Don't forget linear induction motors

This isn't maglev**. Linear-induction is simply the common AC induction motor "unwound" and laid flat" - the copper-coil stator on the bottom of the railcar, the aluminum squirrel-cage "rotor" laid down between the rails. An example is the UTDC system used by the Detroit People Mover and parts of other metro systems. This linear-induction thrust happens independent from any wheel/rail adhesion. It isn't optimized to pull the train car downward toward the track, but it could be.


** Maglev: another technology that moots the gravity issue, because the car floats on a cushion of money. Seriously, cost is the problem: the technical challenges result in very high cost, at least as compared to bulletproof off-the-shelf old railroad, which can get within 5% of maglev's speed. As such, unless cost is no object, I tend to write off maglev.

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    $\begingroup$ A mode that is in use is eddy current braking, which can look like your "second" mode to the uninitiated. $\endgroup$ – Henning Makholm Mar 16 '18 at 2:39
  • $\begingroup$ @HenningMakholm Good one! And it gives me another idea. $\endgroup$ – Harper - Reinstate Monica Mar 16 '18 at 3:16
  • $\begingroup$ OP asked about trams, not trains. The difference is that trams are tightly integrated with road and foot traffic. Ever seen a maglev track? It has to either support the train from 3 sides or let the train straddle the track from 3 sides. It's an obstacle. Maglev is simply no-go for a tram, at any cost. It must be strictly grade separated, suitable only for subway or a train. $\endgroup$ – Agent_L Mar 16 '18 at 10:16
  • $\begingroup$ @Agent_L very good point about maglev. Mind you I never propose maglev, I only mention it to disambiguate it from LIM, (and diss it somewhat). No worries of LIM working on street railways. That said... I'm not entirely sure a street maglev couldn't work via very smart manipulation of magnetic fields, though it would need some magic armwave to keep it from becoming a collector or violent ejector of all things metallic (yes, metallic). $\endgroup$ – Harper - Reinstate Monica Mar 16 '18 at 13:38
  • $\begingroup$ If you pin down the to the rail magnetically, it will already slow down the train by magnetic field emited by the rails $\endgroup$ – Ferrybig Feb 14 at 19:37
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Use Maglev trams and trains. They are free from the issues that wheeled transport would have. Increased braking distances for trains and cars on Mars would be a literal killer.

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    $\begingroup$ Actually, I didn't think of that. :) Good idea, especially as you'd need less EM energy to counteract the train's weight. $\endgroup$ – Tim B II Mar 15 '18 at 0:58
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    $\begingroup$ Not to mention that you need a less powerful magnet to lift a vehicle of the same weight. $\endgroup$ – Nathaniel Mar 15 '18 at 7:25
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    $\begingroup$ Maglevs will also be more efficient since they have even less air friction on mars $\endgroup$ – Ferrybig Mar 15 '18 at 8:08
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    $\begingroup$ Even with all the advantages Mars provide to Maglevs, they're still awfully expensive to build and to maintain. $\endgroup$ – Rekesoft Mar 15 '18 at 12:42
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    $\begingroup$ As we know maglevs currently, they don't integrate with road and foot traffic. They require the track to support the vehicle from 3 sides. Whenever it's a straddle type or tub-type, the track presents pretty big obstacle. Conventional rail needs only two, few cm wide slits. $\endgroup$ – Agent_L Mar 15 '18 at 15:50
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Rollercoasters solve this problem with three wheels around the rail.

This makes switches larger and more expensive, so it would make rail network designers attempt to avoid placing them, and instead build a tighter mesh near city centers rather than joining all the suburban lines to a large trunk, and city-to-city connections would likely get dedicated platforms.

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    $\begingroup$ As short as this answer is, it is right on point, and mentions a simple tech that has been in use for decades. Awesome. $\endgroup$ – Renan Mar 15 '18 at 13:19
  • $\begingroup$ You just beat me to it... $\endgroup$ – T_Bacon Mar 15 '18 at 13:24
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You're right about friction being required for wheel based motion; this is why car tyres are made out of rubber. That said, ever noticed how certain drag cars, bikes, et al can do burn outs?

Friction is necessary for forward movement, but to facilitate forward movement, it only has to be stronger than the forward force of the vehicle. On earth right now, a light vehicle by comparison to the the forward driving force will cause wheels to spin because gravity (via the weight of the vehicle) is outweighed by the forward force. This is also why we talk about a vehicles power to weight ratio.

Just looking at how bikes and drag cars do burnouts though, you'll notice that they way they do that is to apply massive amounts of acceleration in a very short time. This introduces a massive amount of forward momentum (force) that overwhelms the ability of the tyres to grip the road. If you accelerate slowly, you don't overwhelm the friction in the tyres and you can get up to quite impressive speeds.

On Mars, the lower gravity will mean you have 3 options to ensure that you get forward movement;

1) Make the carriages heavier
2) Increase the friction between the wheels and the rails
3) Introduce acceleration and deceleration MUCH more gently

Ironically, the simplest of these might be option 2; the reason why we don't put rubber on train tracks or wheels is that the train is so heavy, the necessary friction is achieved metal on metal. BUT, on Mars, it may be as simple as making the tracks cog-rails, and the wheels cogs. This effectively greatly increases friction by putting most of the pressure of the wheel against the front of the cog rail tooth, and the rear of the train wheel tooth. Theoretically, you could put bands of rubber around the train wheel, but we don't do that on Earth because of the maintenance hassle (car wheels are MUCH easier to remove) and that wouldn't be likely to change on Mars.

Of course, the other option is that you just load up the train. A train that weighs less can carry more mass to generate friction (but physics being what it is, this train will still take more energy to move). That said, the increase in energy required to move the increased mass may be counterbalanced by the lack of atmospheric resistance because Mars' atmosphere is less than 1% the thickness of Earth's. In any event, expect Martian trains to carry much more freight than Earth ones.

Finally, you could just accelerate and decelerate between stations at a much more reduced level as we would on Earth. The good news is that you wouldn't need a very large engine anymore (power to weight ratios again) although it would mean that your train is going to be much slower in getting between short term stops. This is not an option for an inter-city train, but would make much better sense for a trans-continental equivalent on Mars.

This last option has one flaw; the massive changes in altitude around Mars. Earth trains also struggle with this, which is why we make so many rail bridges and tunnels. Mars has much larger changes in altitude between places we'd actually want to go so this may be less of an option and a weak engined train is even less likely to be able to climb a (say) 5 degree incline for an extended period.

So; I agree that rail is still the best option, but exotic wheel options are not necessarily required, and switching could still be preserved with minor modifications to the way train lines work (like light cogging on track and wheel). The one catch with such an option would be that the engineering of tracks and wheels would now have to be more precise and putting pennies or rocks on the tracks would now be more problematic, but that is perhaps commentary for another question.

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    $\begingroup$ Actually, some rapid transit systems do use rubber tyres, which I noticed just now. Regarding weaker engines making it even harder to climb inclines, wouldn't the lower gravity compensate for that? $\endgroup$ – Sean Mar 14 '18 at 23:33
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    $\begingroup$ I like your cog-rail idea. $\endgroup$ – user253751 Mar 15 '18 at 5:29
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    $\begingroup$ Instead of loading up the trains to make them heavier you can just reduce the number of wheels per wagon. If you reduce your 8 wheels wagons to just 4 wheels, you have just doubled the friction between wheel and rail for the same wagon weight. $\endgroup$ – Rekesoft Mar 15 '18 at 12:46
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    $\begingroup$ Why not just have assisted start and stop! Like a rollercoaster launch. The force needed to maintain speed would be much less. $\endgroup$ – Jammin4CO Mar 15 '18 at 13:59
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    $\begingroup$ @Rekesoft not exactly. You have doubled the contact pressure but also halved the contact area. The total force (both the normal force and friction) remains the same. Your modified carriage won't move faster, just wear out faster. $\endgroup$ – John Dvorak Mar 16 '18 at 0:58
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Rack or Cog railways would not depend on gravity as much for their friction, and you could guarantee contact with between the cog and rack by having wheels above and below the rail.

https://en.wikipedia.org/wiki/Rack_railway

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    $\begingroup$ It is a surprisingly little known fact that friction coefficients can in fact be arbitrarily large (limited only by material properties), since you can mesh the road (or track, etc) material with the wheel such that the road would be sheared off in case of slippage -- like gear tooth breaking off. Or simply note that you do not need to rely on gravity on the equation Friction = Normal force * friction coefficient. You can compress the tracks from the side with arbitrarily large force. The greater problem is for track-free land vehicles however, that rely on the integrity of the soil itself. $\endgroup$ – Real Mar 15 '18 at 23:54
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consider Monorails.

All the switching problems were solved long ago, and the wheels are applied to all sides of the rail. In some designs the load bearing wheel is not the wheel providing thrust or braking, the side wheels (or a track brake does) that, which exist in opposing pairs so they are not affected by the reduced weight.

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enter image description here

It can have an inverse triangle shaped track. This allows rubber tires on top to carry the weight, and tires on the beveled sides to guide it.

But much more, all the tires would have airbag suspensions, allowing the wheels to press on all three sides of the triangle in precisely controlled ways. While the top ones would bear the weight of the monorail, all three tires would also be opposing each other, effectively clamping the rail.

These extra "opposition forces" create artificial downforce-on-rail that doesn't depend on the weak gravity. This extra adhesion is just as usable as gravity-derived adhesion. Better yet, monorails tend to use rubber tires on concrete rails, so adhesion is comparable to an automobile, at around 100% rather than the ~40% of steel-on-steel.

It should be able to brake quite well, as much as two "gees" on Earth and quite a few local "gees" -- which may be too much for the safety and comfort of those acclimated to the weak gravity, especially if (like most trains) it doesn't have seat belts. Earth transit systems prefer to limit accel/decel to 4.5 mphps which equates to 2.0 m/s2 or about 0.2 gee.

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  • $\begingroup$ Hah, I just posted the beginning Monorail lines as a comment to OP. $\endgroup$ – sirjonsnow Mar 16 '18 at 12:42
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Just to add my two cents here: less gravity would give some simplicity if compared with regular (Earth) trains/trams.

  1. Number of axis. Talking about railroad, most likely you imagine a carriage as something with two bogies per carriage, two-axis each. This is done to decrease a pressure applied to rail. Lower gravity allows you to come back to one-axis bogies (two axis per carriage at all, as it was in XIX century) because you don't need to distribute that much pressure with multiple touch-points.
  2. Carriage size. With less gravity, you can increase a carriage size thus efficiency (make it two-level, wider, longer etc.) having the same pressure on the rails. Even you can do it as a flex-jointed sections, one axis per section.

And all this not about electric-magnetic field, just old good mechanics. With electricity involved, you can move far beyond this point.

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