I think you'd be really hard pressed to find a working class person who can live and work car free. Public transit only goes to really high end businesses that can afford the expensive boston/cambridge office space, and doesn't run at all for people doing shift work. People who work at building sites, do in-home work (cleaning, trades, etc.), or who work in warehouses will need a car.

Traveling outside the city without a car is also basically impossible, so you have to live your life inside Boston and Cambridge, or pay thousands of dollars in rental fees to use a rental car when you want to leave.

The people I know who are car free in Cambridge are overwhelmingly high income and spend a lot more on transportation than I do.