The 99-B a pretty low-cost bus, because it's frigging' crush-loaded much of the time. A subway would be much more efficient, though. A streetcar would have the same problems of capacity as a bus - in fact the peak of peak would get worse because the traffic overwhelms both the 99-B and some of the relief routes that wouldn't be served by a streetcar, like the 84.
Here is the current set of alternatives discussed:
http://www.translink.ca/en/Plans-and...e-Designs.aspx