Another solution is to use existing underground ways or create new ones.
Due to the narrow width of the vehicles and the narrow tracks, the underground infrastructure can be in covered trenches or in reduced-dimensions tunnels. The solution can therefore be built for a much lower cost and a much faster, than any classical public transport solution.
It is even possible in a tunnel with a metro diameter, to create 4 two-way traffic lanes in a single tube, as in the DUPLEX A86 to the west of Paris, allowing:
• a significant increase of the capacity : up to 12,000 vehicles per hour (i.e. more than 6 motorway lanes! per direction of traffic) and potentially up to the capacity of a RER of 60,000 people/h per direction.
• dedicating a high speed lane, which will allow for example to cross a large city in a few minutes: 10 minutes or less instead of an hour to cross a city of 20-km diameter, and without any impact on the city.
As much as possible, the stations are carried out on the surface to avoid any stairs, escalators or elevators. The vehicles are going up the service roads to pick up and drop off passengers.
If the major drawback of underground ways remains the cost of infrastructure, they can bring a lot, especially in old cities or in highly urbanized areas by preserving ground surfaces and reducing impacts.