I’ve always been fascinated by the grand civil engineering challenges. Our cities are still so much the products of centuries-old thinking. However, sometimes the new ideas have non-obvious challenges too. That’s one reason they weren’t tried already.
Take the gravity-train. It has the benefit of needing no engine, saving massive amounts of energy otherwise lost to braking and accelerating huge masses of metal and meat. Gravity (and some occasional lifting engines built into key sections of track) can do all the work. Put the stations at the high-points and the trains will naturally start slow, accelerate, decelerate and stop at the next station with very little energy. Viola. (ignore the picture above — there’s no good reason for that camelback hill except for fun).
If you think about it, the fastest and most efficient way to get from New York to Tokyo is not a scramjet flying to the edge of space, but rather a straight line tube through the earth.
Such a tube would want to be sealed at both ends to maintain vacuum against atmospheric pressures and more importantly lower drag. It’s the vacuum tube delivery system from my childhood. I loved that toy. I think Costco still uses these for sending stuff from cashiers to the office.
Relative to the surface (i.e., if you unrolled the planet to make it flat), this tube or track would be curved, just like a rollercoaster valley, with the train cars accelerating “down” until the halfway point and then decelerating back “up” the hill to gently stop at the destination.
Gravity would be significantly less at the low-point, depending on the actual distance you’re bridging. But who cares? You have bigger problems if you ever dig below the earth’s crust.
Such a system would require very little power, once built, just enough to overcome any friction, and maybe an emergency drive mode. I’d expect much more power required for active cooling than any acceleration.
For the much smaller city-sized version, you could also employ the same underground gravity train technique, if you’re tunneling subways anyway. Why not?
Well, there is always the logistical issue of multiple trains on the same track…
Think about it. One needs a way to stop, wait, and start trains almost anywhere, including, in the worst case, on a hill. Trains get stuck at stations, or worse. Police interventions, accidents, etc.. can cause these kind of delays. Just consider what it means to build a walkway along the track for emergency evacuations, etc..
Themepark rollercoasters solve this by having special flat sections of track where the cars can be braked and later re-accelerated — generally one of these areas for each car you want to run simultaneously, so they can all stop, if necessary, without collision.
They also, if you didn’t notice, have places where you can get out and climb down in an emergency. And maybe most importantly, they have people whose main job is to walk the track every single day looking for weaknesses.
Still, it would be a lot of fun, don’t you think?