At a debate last night Ken repeated his ambition to replace the buses on Oxford Street with trams:
If re-elected I intend to start installing a tram running the length of Oxford Street. There are some factors which pose difficulties but we have thought about it and there are ways of overcoming them. It will either be a shuttle tram or will be linked to the Cross River Tram
We would demolish the gyratory system [at Marble Arch], handing part of it back to green land and building two developments on part of it which will fund the project. People getting a taxi would have to use the roads running north and south of Oxford Street. We would phase the buses out slowly and maybe run some along Wigmore Street.
You can find a few more details in this Evening Standard piece, which mentions a £200m cost and a 2012-18 timescale. The article quotes statistics from this New West End Company press release, who sponsored the debate, and are major backers of the proposal. Ken discussed much the same idea in 2006.