We've done this debate to death. Third rail isn't an easy win because 1. The power infrastructure around it isn't simple. 2. You still need to actually have somewhere to put the third rails (e.g. Sleepers with the 'pots') and 3. The railway authorities don't think it's safe.Third rail in the South East - for example to Uckfield and the Ashford - Hastings line. Seems a no-brainer.
Point 3 is a matter of some contention, which I don't wish to restart here.