Well this is already ongoing. In France:

We already have automated (no staff in train) metros (Paris' 14, Paris' 1, soon Paris' 4, Paris' Orlyval, Rennes' a/b just for the ones I've used)

I remember there was a prototype bus (12 person size ~) at La Défense - with a staff always on board, and I see that now there is "EZ10" in Toulouse, but I remember seeing also other lines as well. I'm pretty confident there are many all across the world.