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:
- 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);
- 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;
- 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?