Don't be so sure - one of my clients works for Network Rail and it is being studied. Dual voltage trains are not an issue - they are already used on the network with trains that originate in Brighton and then go onwards to the north of London. Eurostar still operates on dual traction as well into Waterloo.

Of course it would have helped if the HEX had a spur tunnel out towards Reading or on the south side of the loop out towards Ashford where a quick link into the Southern region and hence Waterloo and Clapham junction would have been possible.

I have posted before that there were plans at one point for a dual rail track running in between the anticlockwise and clockwise M25 carriageways, certainly between the M23 and M40 sections. This was to create a kind of "transport corridor" like you see between Schipohl and central Amsterdam where the trains run in between the carriageways of the A4 motorway. A high speed LGW-LHR link was studied with a connection time of <40 minutes. Sadly the Thatcher government believing the car was the future and trains were archaic killed it on cost and the M25 was not built with enough central reserve to allow it in the future.

A bit like the other legacy from Thatcher; the destruction of the coal mining industry. With gas now rocketing in price (I do recall this being fully predicted at the time as North Sea supplies started to run down) the coal fired power stations are now making a comeback. Although we now have to import most coal from Poland and Australia despite having >100 years supply underground in the UK.