First point. Ice. If a system has track that is directly exposed to the elements anything but LIM propulsion would seem to be problematic, especially at higher speeds. With a hanging system though, the track may be expected to remain dry except for condensation. If the running surface is to be rubber-mounted for sound and expansion reasons, however, its thermal mass is so low that it can easily and cheaply be kept at or above ambient air temperature, with a low wattage resistive wire, so that condensation would not form. Let me be clear. This is not about melting ice or evaporating moisture. Condensation will only occur if moist air is warmer then the track surface, such as if warm ocean fog is contacting track that is still cold from the night before. There are several points worth considering. One is that all activity by vehicles using the track will generate heat, and lot’s of it. Actually getting rid of the heat in (in a hanging system) would seem to be the greater problem most of the time. A second thought is that if there was a condition promoting condensation, care should be taken to design the track so that water will not drip onto the running surfaces.
About smooth surfaces – One thing that has been pointed out is that smooth metallic running surfaces do not provide much traction in the first place. I would offer this. There is only so much braking that you want to subject a passenger to, except for emergencies. I submit that on a smooth dry surface there will be sufficient traction to reach the braking limits that would be acceptable from this comfort standpoint. So the traction issue is a safety/extraordinary event issue that will never happen anyway.
All brakes, including linear and rotary motor magnetic braking systems, will be overwhelmed by being undersized, and they will all certainly be undersized to handle “brick wall” stops instantly, traction or no traction. For this reason a back-up system is needed. The obvious solution is to directly engage the track with brake shoes in some manner. Such a system would not be used for routine braking, lest there be wear to the track over time. But for emergencies, extreme braking power is quite feasible.
This brings up another issue. Passenger restraints. I personally favor a padded waist restraint bar if it can be incorporated gracefully. I would like to discourage movement about the cabin, so that it (the cabin) can hang semi-freely, rather than have this motion be 100% simulated. We don’t need kids intentionally “rocking-the-boat” so-to-speak.
There is another safety feature that is sometimes mentioned which bears consideration. There is no reason to supply power to the track directly behind any vehicle. Having no power would seem to be a pretty good defense against malfunctions of headway distances. I can’t say that I have worked out the details, but it is worth noting that if there is no electrical draw, switching on and off can be repeated with very little wear. (no sparking, etc.) I would like to see what inventive minds could do with the concept.
I do not think it is unreasonable to consider that each vehicle should have several rangefinders to determine how it is spaced between others. Such devices are commonplace these days. They come with cameras and are showing up on cars as a way to help drivers not back into things. With a homogonous fleet and guaranteed vehicle-to-vehicle alignment, I think self-spacing and impending-collision detection systems are very doable.
With a hanging system, safety should be almost a none-issue. In post 43 I illustrate how the bogies from which the cab hangs can be spaced to prevent those cabs from hitting each other, and how the swinging action can help absorb shock. Another idea would be a sort of airbag idea for bogies, shown below.