In the 1970s the USA searched for the best public transit option and Personal Rapid Transit (PRT) won in the form of "Cabintaxi" -- a German/US venture that was fully developed, licensed by the German transit authorities, fully safety tested but when the US pulled out and demanded higher NATO fees from allies (including Germany) the German government dropped the project due to budget concerns. Detroit, of all places, was pushing for Cabintaxi urban transit but dropped it when the US government pulled out of the program.
More info, pictures, a video and study related info at the link.
Cabintaxi was designed to handle multiple vehicle sizes:
* a 4 passenger "personal" taxi
* both a 12 and 18 passenger vehicle
* to increase total riders on busy routes the 12-passenger vehicle can be linked with another 12-passenger unit
* likewise, the 18 passenger models could be linked with one other like-sized vehicle
* and finally, a version designed for standing passengers only, no seats. I get an image of the tokyo subway system when I think of this.
The track was also designed to have vehicles both on top and below (suspended from) the guideway, doubling the capacity. This makes Cabintaxi the most flexible mass transit system ever designed. Its numerous variations gave Cabintaxi far more technology adaptability than any other urban transit technology ever developed.
Why are we stuck with 100 year old technology (buses and light rail) when Cabintaxi is both more convenient and more flexible -- and isn't dependent on draining funds from the tax payers just to stay afloat.