So, lets put the challenge this way: what if making the Charles River more /transitable/ weren't about boats, but could replace some of the Charles River Roads with transit?
We've talked about:
Green Line on a Reduced Storrow (NS spur to Kenmore)
Kenmore to West Station
West Station to Anderson Bridge
Anderson Bridge to Greenough or Soldiers Field Road
Greenough / SFR to Arsenal St
Arsenal St to SFR/Beacon
SFR/Beacon to Watertown Sq
The interesting thing is that whether you think of these as bus or rail, they *still* have the problem that station walksheds are semicircles and that the river wastes the "swimshed" side