The way it works today is that there's a frequent bus on each of the major east-west streets (4th, Broadway, King Edward, 41st, 49th). The capacity provided is inadequate and the speed is substandard, but a mixed-traffic streetcar would scarcely be an improvement.
I just wrote a long blog post about why a full SkyTrain extension is the only solution to the corridor's transportation needs:
http://pedestrianobservations.wordpr...train-and-ubc/