In question time with Mayor Boris yesterday he said: "We have got to improve the transport links in south London and we are looking at that now." Given that (having scrapped a couple of bridges and a couple of tram projects that would have helped) an extension of the Bakerloo Line is one of the most realistic ideas on the table and there was speculation in thelondonpaper yesterday that this is what Boris has in mind.
The project is uncosted and no preferred route has been identified, so an actual functioning line would be many, many years away.
Still, let that detail not stop us.
Above is a route preferred by the sort of people who enjoy discussing these sorts of things on specialist websites, heading south east from Elephant and Castle to Lewisham, via Peckham and New Cross Gate. This route would have the advantage that it would integrate with existing south east London transport hubs and serve a stretch of London with enormous regeneration potential (particularly along the Old Kent Road).