This will slowly become a sensitive topic with the pressure against diesel traction. The ORR doesn't like the idea of extending third rails, so that is a nonstarter for Worting Jn / Redbridge to Salisbury and Exeter. Is it worth prodding the DfT to say that 800 series bi-modes won't really be suitable either?
This article should provide some background info on power sources - https://www.railfuture.org.uk/article1825-Electric-alternatives
Thanks to Railfutures for this interesting article.(Perhaps reinvent the TC principle - trailer coaches which can be propelled by electric units/locos and then change over to non electric traction at the end of the 3rd rail?)
Well it won't be happening for a while, until the next franchise, - but I thought the best units would be Bi-Mode Stadler units, similar to greater Anglia's, but on third rail. The only problem is the diesel engines would take up valuable platform space.