There's a fourth, the Green Line (which actually has 4 branches, B, C, D, and E, on the south/west side of downtown Boston), but it is not shown in the article under discussion. There is a link in the article to another page on the same site that shows the Green Line.
There's a fourth, the Green Line (which actually has 4 branches, B, C, D, and E, on the south/west side of downtown Boston), but it is not shown in the article under discussion. There is a link in the article to another page on the same site that shows the Green Line.