Also, the third rail effectively doubles up as the DC distribution system along the line, connecting all the feeder station outputs together. As was pointed out in last year’s discussion about this hypothetical dynamic switching, you’d therefore need a parallel, but insulated, circuit separate to the existing track system, but still passing the full traction current. To avoid voltage drop it would need a huge cross-section. Not only would switchgear get hammered as you say, the sheer amount of it required would probably be unaffordable.
All depends on the granularity of the switching required. I don't see much benefit in conductor switching on UK segregated plain track. On the long single lines that might be considered for future 3rd rail extensions just using block switching would result in conductors being live sometimes many tens of minutes before a train arrived, and high block occupancy would lead to them being energised most of the time anyway. The main place where switching might be worth considering is in station platforms. If we imagine our battery-equipped train coming into a station, leaving the usually energised plain line conductor a fair way out of the station at the home signal and running through non equipped throat paintwork on battery. The conductor rail recommences once the platform is reached though remains isolated during this run-in. Only when the train is completely stationary is the platform power switched on and that remains energised for the train to 'take a drink' if necessary and assist in subsequent acceleration. Once the train has completely departed the platform, the platform power is disconnected until the next arrival. That would remove a large risk in the number one cause of passenger deaths on the UK railway in recent years, falling from platforms, often when there are no trains about. As the train leaves the station it encounters the normally energised plain line conductor rail a little way out and continues as normal, taking power for traction and in motion charging as required.
It would be a total nightmare and practically impossible to section up the throat at Waterloo lets say and switch short conductor lengths on and off according to signal route setting. My solution would remove all that throat conductor provision anyway with its short lengths through crossovers and miles of complex jumper cables. All the trains need at least a moderately sized battery first though.
I suggest that examples of low speed, relatively low current, short distance city centre tram systems just won’t scale up to a busy main line heavy rail system.
That's a specific risk where electrified infrastructure is routinely sharing space with road vehicles, pedestrians, animals. In such localities, no exposed conductor must be live that can be touched by any member of the public and it's not a new idea. It was the principle behind the short lived Dolter stud supply system used initially for the early 20th century Torquay trams (and Hastings), as influential locals didn't want unsightly overhead wires. Metal studs were embedded in the road at intervals, connected by underground supply cables. A long pick-up sled under the tram was magnetised by traction current or a backup battery aboard. When passing over, the electromagnet attracted an armature in the stud box under the road that threw contacts to connect and disconnect the power at the stud. Needless to say, reliability was a problem and crews were equipped with a large mallet to bash a protruding indicating nipple on the stud boxes, connected to the armature, encouraging the contactor into its desired position. If it failed to disengage after passage of a tram that would lead to a high voltage being exposed to the public including the metal shod horses still common on the roads of the time. After some incidents, an alarm bell was devised onboard that would alert crew if a stud was uncovered still energised after passage of the vehicle and the driver was instructed to stop immediately and go back to give the box a good bash with the mallet. When the tramway wanted to extend along the coast, Paignton Town Council refused to permit road studs so they were forced to use trolley wires and decided to convert the entire system in Torquay too.
