“How many above ground, non tunnel, exposed to weather miles are in the MBTA system?”

I don’t have an exact, official amount for that, but I did some quick math to get an approximation. The MBTA subway system is 64 miles in total, not including the Silver Line. Of that, about 27 percent of the system is underground, so says Transit Maps of the World (an excellent, excellent book). So, if we multiply 64 by 73% (the percent of above ground trackage), we get 46.72 miles. There you go!