underground roller-coaster

London underground lines have the tunnels exiting stations running on a downward incline until the midpoint between stations and then upwards to the next station. They do this to make the trains run more efficiently by converting potential to kinetic energy and vice versa as the train goes between stations.

The logical extension of this idea is the underground roller-coaster, in which almost all the energy of motion of the train comes from its vertiginous descent once it leaves the station, which is then almost entirely converted back into potential energy when it reaches the next station.

At each station the train will still be moving, although fairly slowly. There are all sorts of health and safety problems with getting people to step from a moving train onto a stationary platform, so to resolve this, passengers will step from a moving train into a chair on one of a number of slowly rotating underground Ferris wheels.

When a Ferris wheel chair reaches the top of its rotation (underground, but near the surface), the passengers can step from this onto a slightly slower moving walkway; stepping off this as it approaches the station exit will be no harder than stepping from a moving escalator.

The only challenging part of this design is that creating the swooping curves of the roller-coaster tunnels may need a redesign of traditional tunnel-boring machinery.