So this is something I've been trying to address all thread, so excuse me picking your comment.

You can only create a public transit platform by having _dedicated_ infrastructure. We know this because cities like London and Paris (and not-so-big Vancouver) all have mostly-to-fully automated trains with 90-second headways between each train at rush hour. This basically means that if once a train pulls out of the station, the next one pulls in mere seconds later. These trains are usually packed to the gills. Even one single-occupancy vehicle would decrease the overall efficiency of that system.

Heck, you can create a tunnel in each which sled contains one bus and it'd still be less efficient than the London Tube because the whole thing will take up more space, physically (these trains squeeze in every last passenger). This is what Seattle tried to do with the downtown bus tunnel and they're evicting all the buses there in favor of light rail.

I think it will always make sense for subway trains in dense city cores, but I think the discussion about dedicated rapid transit is hampered in more suburban areas because of the whole notion that we have to fit in cars into rapid transit corridors, or else it's not worth it. But this ends up making buses less reliable and gives them their crappy reputation (see: https://en.wikipedia.org/wiki/Bus_bunching).

Grade-separation is absolutely necessary for good rapid transit.