There is no "average" or "standard". You should either consult with a local real estate agent/broker or work up comparable properties to arrive at a market value for a property, one consideration would be the added/decrease in value due to the T and it's proximity. You might also look at the values of the properties all along the line and compare/contrast their values to those say across the street from them. Because of the differences in all the properties you may be able to discern a value that may or may not be quantifiable as the value you desire.
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,