I just want to contribute to this thread because it's the first hit you get when you google Quincy to Cambridge on bike. For one thing the Hancock St bridge aka Route 3A bridge aka Neponset River bridge now has a wide sidewalk for bicycles.

I commute Quincy to Cambridge everyday and have grown so frustrated with the Redline experience that I am planning to figure out a way to bike it safely. I will start trying to do it over the weekends and if I make any progress I will post tips on this thread.