Or perhaps to put that another way: In a city with great public transport, lots of the person-miles are done on full trains, running back to back all rush hour

Or perhaps to put that another way: In a city with great public transport, lots of the person-miles are done on full trains, running back to back all rush hour, in a way that nothing else can really replace. But for the system to be useful, it has to maintain reasonable frequencies until midnight, out to a lot of points. This results in a lot of vehicle-miles being done almost empty. But they are really essential. It's the second class which maybe small automated vehicles could replace. Lots of cities only cover the first class, and wonder why so many rush-hour commuters don't use it...