"Christopher A. Lee" wrote

Of course it made sense to use the existing 4 rail system because
apart from the LBSCR's overhead AC that was what the other London area
lines used. The LNWR electrification was planned in 1907 and opened in
1914, with the Bakerloo linking up a year later.

The original plan was to extend the New Lines from South Hampstead in
deep-level tube to a terminus below Euston. In 1911, before the New Lines
opened, this plan had been abandoned in favour of linking with the Bakerloo.

The LSWR 3 rail system was planned later with the first section
opening in 1915.

Did the LSWR ever consider using the 4 rail system? The first LSWR line to
be electrified (apart from the isolated Waterloo & City) was the route
between Waterloo and Wimbledon, via East Putney, including the section
between East Putney and Wimbledon which had already been electrified on the
4-rail system for District trains. So this was the first use of a line
adapted to take both 3rd rail and 4th rail trains.

Peter