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.
Technically two or three of the silver line buses are also considered to be subway, too. I believe SL1, SL2, and SL3. They’re subway fare and they go underground at some point
And free outbound from Logan! Though I must say, the last time I did this it was a bit of a nightmare. The SL1 was very late and we were packed in like sardines.
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.