Some profs over at the University of Washington have done a report on how the streetcar network in Seattle can expand into 5 different neighborhoods. Their proposal doesn't give costs but lays out the amount of development that could possibly contribute to a local improvement district (LID) and TIF to pay for the expansion. The corridors would connect a larger part of the downtown to the light rail system thats currently under construction and provide a push for development close to downtown. Very interesting stuff. The Seattle Times has more. The graphic below shows the lines from the study.