The T has many problems, but I don't recall any specifically heat related. To the best of my knowledge all the rail vehicles are air conditioned. I don't recall any open windows since the PCC car days. Okay, perhaps the Mattapan trolley (still classic PCC) might get a bit warm but it is still worth the effort.
If you are really a hard-core transit nut, you should order a copy of
https://www.amazon.com/gp/product/0262048078
I know the author, who runs an amazing map shop in Cambridge. The book can only be described as excessive. I don't recommend carrying it around on foot!