When the T Third street line was being built they started construction on a loop from Third street on 18th street to Illinois (parallel to Third which still has abadoned freight train track) down to 20th and back to Third...

There was some sort of dispute with the Port of SF I believe about the loop....not sure exactly why... maybe the port wanted the freight track kept in place but that's no longer feasabile.. or it might have been lack of money to complete it.

I understand the city now has federal funds to complete the loop.... That part of Mission Bay has a new UCSF hospital and lots of new housing under construction so they'll eventually need some E line service to Mission bay....

I've seen a Mission Bay Destination sign on some of the historic street cars