Last month the Elizabeth line opened for service in London, one of the things that makes it different to most the existing lines is that the trains use AC power instead of DC. Most of the Metro trains in London that I am aware of use DC power because the lines are very old and in the olden days they didn't know any better.
The problem the DC power is it is hard to divide it up without producing a lot of heat and wasted energy. The other problem is you can't have two trains on the same loop because on DC power, one train will cause the power of the other to surge or sag depending on what the train is doing. The other problem is you can't use high voltage because the whole current in the loop flows into the train.
To solve the problem they put the a physical break in the live rails so the brushes pickup the charge for a while then they are moved onto a different DC loop so they don't share the power supply of other trains. I think each loop is only 630volts.
The Elizabeth line trains use AC power, the electrical current is changing direction 50 times a second so it can go through a transformer on the train and a smaller charge is taken. The power is divided up so a small charge is taken from the current and then converted to DC. The power is divided up without producing much heat.
All the trains can share a single loop because they all have transformers that divide the power up so most of it goes straight back into the overhead cables.
