Yeah, the closest stop on the Plymouth line to Cohasset is Quincy Center. At which point you might as well just get on the Red Line instead of the Commuter Rail. In order to actually get there from Cohasset without driving you have to take the 714 bus (not part of the MBTA system) to Hingham Center and then catch the 220 bus into Quincy Center. I spent a summer doing this to work at a non-profit in Cambridge before I had a car. And the 714 I don't believe ever actually enters the limits of Cohasset... I just happened to live on the side of town closest to Hingham and Hull so it stopped near my house. Cohasset is very snooty and probably thinks public transportation is "lower class". I think I even remember getting into that mindset myself. But I got over it!