[pas]

The problem is, that it's veeeery low density if only a few people sit in one car.

Sure, with increased efficiency and better energy economics, that's not a problem.

[cracell]

Couldn't you just make smaller trains that fit in the same tunnels?

Sure tons of cost for R&D but just because the tunnels are smaller doesn't mean they could only be used for cars in the long term.

[adventured]

That appears to be the intention of sorts. Sleds that hold numerous passengers (8-10?).

You can see one in the CG demo, in the first few seconds of the video, on the right:

https://www.youtube.com/watch?v=u5V_VzRrSBI

[pas]

also at 0:48

[pas]

And rewatching the video, I wonder... currently a subway / metro car holds 50-100 people (and there are about 5-7 carriages linked), but subways follow each other at best 3-5 minutes. (Or ~1-2 in Japan in rush hour.)

So about 250 - 700 people are moved in 3 minutes, that's 80-230 per minute. If sleds follow each other by let's say 2-3 seconds, and each car holds 1-8 people, then that's 20-240 people per minute. With the added benefit better granularity, and you don't have to switch lines, metros, you don't even have to leave your seat.