A Transport Group has been set up to combat congestion on the A40. Some of the proposals that it would like to look into include restoration of old rail line from Oxford to Witney for trains or trams.
http://www.oxfordmail.co.uk/NEWS/10931531.Railway_line_could_hold_answer_to_A40_problem/
Update – 17th January 2014
The launch meeting ended with a unanimous call for a feasibility study to be launched. In addition to Tram Trains and Trains, the meeting also suggested that a guided busway could be an option.