Steve Tripoli of member station WBUR reports on the introduction of car sharing programs in Boston and other cities around the country. In Cambridge, Massachusetts, for example, customers pay a yearly fee and hourly and mileage charges. In return, they get access to "common cars" when they need them. Car sharing companies have also been started in San Francisco, Portland, Seattle, and Chicago, and more are likely, if the concept becomes profitable. So far, it isn't.