I don't understand why subways and trains aren't implementing. The universe of possibilities is so incredibly limited in these situations that it seems like a piece of cake, compared to city driving.
"Can you see that the track is clear of obstacles? If not, stop!"
They are, often on a huge scale. The DLR[1] in London is entirely automated (although sometimes the ticket inspector will sit at the front with the backup control panel open, presumably so they can pretend to drive a train), and the Central, Northern, and Jubilee lines also run semi-automated, and I'm pretty sure the bits that aren't automated are due to union lobbying rather than any particular need.
"Can you see that the track is clear of obstacles? If not, stop!"