Modern ERTMS involves the system knowing where all the vehicles are; a centralized system will be more efficient than a distributed one if all the information is available. I don't think that's a fair characterization of British train disasters; the only one I can think of that involved a signalling failure is the Clapham Junction crash, in which the wires were connected wrong - and connecting the wires wrong will break any system.

A vehicle the size of an 800-person tube train wouldn't be able to get round the tube corners without rails, and how could you negotiate a destination that made sense for that many people?