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.

[1] http://en.wikipedia.org/wiki/Docklands_Light_Railway