When I was recently at West Ealing I saw GWR’s battery powered electric train. It does not rely on a third rail or overhead wires.
In some ways this is a recent thing, but then again, it isn’t. The first experiments with battery trains were done nearly a hundred and forty years ago in the 1890s.
BR (British Railways) used batteries in an train from 1958 to 1966 on the 38 mile long Aberdeen to Ballater line in Scotland.
GWR have been experimenting with a battery train on the West Ealing to Greenford line in West London.
When working in London I sometimes commute from West Ealing to central London, and I had seen the inductive charging between the rails.
On a recent visit though the GWR battery powered train was at the station. The GWR battery powered train is a converted Underground train, and has been used for testing, but doesn’t carry passengers.