I have to confess that I haven't really followed the modern metro developments of recent years. It's been a while since I read the details on this, but I think the 1500V DC overhead systems were phased out on British Railways lines by the 1970s or thereabouts, but they may well have been re-introduced on some of the newer mass-transit systems.
I guess I should've mentioned the old "tram" (U.S.=trolley) systems, but it slipped my mind. I've been on the Blackpool trams and the line down in Seaton, Devon.
They were gone before I was born, but it's also worth mentioning the "trolley buses." They had electric traction but ran on normal rubber tires on the road to give greater manueuverability. Pick-up was by two overhead arms.
I've not been able to find a definitive answer on the 660 vs. 750V issue. I've seen older books which quote 660V for the whole of the old Southern Railways area. I'm wondering whether increased traffic meant that at some point they decided to raise the supply to a nominal 750V to allow for greater voltage drop and just regulate it down on-board. I've tried a few web searches, but not found any answer to this.
On the Underground, again I'd like to get a definitive explanation of the development, but from what I've been able to piece together, it seems that most of the network now runs with +420V on the 3rd (outer) rail and -210V on the 4th (central) rail to make up 630V nominal.
Where tracks are shared with 3rd-rail only trains, they apparently revert to full positive supply on the 3rd rail and bond the 4th rail to the running rails and ground.
Some parts of the Underground used other methods years ago. The Central line originally had just a central 3rd conductor rail with return via the running rails. I think it was converted to 3rd/4th rails during the 1950s, if I recall correctly.
One of the other Yerkes tube lines (Bakerloo I think) originally reversed the polarity on the rails due to leakage to the tunnel walls, but was later changed to the normal arrangement.
I'm not even sure if the split 420/210 supply has a solid ground on it or whether it might be resistance grounded. It seems to be remarkably difficult to get the full technical details anywhere.
