I don't think I'm advocating unsignalled. Just put the signalling into a distributed system into the trains themselves. Since those vehicles can actually see what's in front of them, that should be a lot safer, since signalling systems are effectively blind (that's where the disasters tend to happen. The central system wasn't aware of, say a broken connection that put a switchboard into an unexpected position).
I'm convinced this can be made safer than central signalling.
As for the vehicle sizes, I'm sure whatever size vehicle is necessary could simply be constructed, and the article on wikipedia for mentions that you could have battery-powered vehicles with rapid charging at stations, negating the cost of having and maintaining dynamic (moving ?) electricity connections throughout the tunnels.