On Sun, 17 Apr 2016 15:27:40 +0100, e27002 aurora
wrote:

Would these routes have utility today? Absolutely. Given London's
desperate need for relief to its overflowing population, picture this:
A 25kV Chiltern upgrade from Marylebone taking over the TfL fast pair
north of Harrow-on-the-Hill. Beyond Aylesbury there is upcoming
development at Aylesbury Vale.

Given modern control system and multi-voltage trains, it might be more
economic to lay down a third rail from Marylebone to Harrow, while
retain the fourth rail between there and Amersham the same way other
section which run both 3rd and 3rd/4th rail together.

This would avoid the cost of raising the tunnel between Marylebone and
Finchley Road, and of rebuilding bridges.

This is also something I have suggested for some main line situations,
too - 25KvAC in the country where they can use the power for speed,
and 3rd rail in the cities to avoid the cost of major infrastructure
rebuilding.