AKA "the purple line" the commuter
rail serves the suburbs
outside the city of Boston.
The commuter rail is not nearly as good as the T, in that it is rather slow, there are not a lot of trains (2-3 per hour during peak hours), crowded during peak hours (otherwise empty), and only moderately well cleaned.
Also, if you are tall, the regular seats don't have good legroom, so your legs may fall asleep on rides longer than 20 minutes. However, if you sit on the fold-down seats at the ends of the car there is legroom aplenty.
The good thing about the commuter rail is that it can be very convenient if you live in a town that the tracks go through. The commuter rail has good range, too- it covers most of eastern Massachusetts, so it can be better than driving if you are commuting to Boston