Having the T abutting your property is a subjective value. It would be a value in the eyes of a potential buyer. To some the noise etc. may be a negative, while living in close proximity to a public transportation station/pick up spot may be a bonus. It's like when people ask how noisy is Logan Airport, it's really a subjective value. To someone who comes from Springfield it may be noisy, to someone who lives in East Boston it's just background noise, it's all in what your experiences are and what you are looking for.
It's kind of a non-answer answer but I hope it helps,