wrote

I believe the stretch from Queens Park to Harrow & Wealdstone is still
four rail, otherwise Bakerloo passengers would have to get out and
push!

Ditto Gunnersbury to Richmond for the District Line. However, on these joint
sections the fourth (centre) rail is at neutral voltage, with the third rail
at +750(?)V, whereas on the rest of LUL the fourth rail is at negative
voltage and the third rail at positive voltage - i.e, even apart from
loading gauge considerations, a LO or other National Rail DC train would not
run satisfactorily on LU lines.

Peter