Two of my friends live in the Fenway area with an 18-month-old. His stroller is collapsible and has a car seat that comes out of it. They take the train nearly everywhere and only take Zipcars when visiting family out of town really. They also bike with him and walk with him to appointments and daycare. Hell, my friend walked to the hospital to have him last summer.

They often carry the stroller up and down stairs at stations without working elevators (I've helped before). They specifically have a stroller that is rather light. I believe they are planning to get an ebike eventually when he's big enough to sit in the back bike seat. It's super doable, but they are also very patient with a non-fussy baby. They are an active, healthy couple who are able to do things like carry the stroller up and down staircases and walk 20/30 minutes to daycare. I think it's doable if you are already used to commuting (neither of them have a car, nor want to own one in the city). The Fenway area is also well-connected, and everything they need is within a 30 minute walk. If they weren't so centrally located, it may be a different story.