The mbta subway is the oldest in the usa and the lines are relatively short so you could add both they blue line and red line or blue line and orange line. The blue line is special as it switches between third rail and overhead wire. The only metro in the USA that does so. It would be a great addition to the game. And with the mbta having not too great frequencies it would be easier to add. And more than half of the system is under ground.
More information like characteristics of the network and what type of rolling stock is used, length of a line etc
for the red line you got the 01500 series 01600 series 01700 series those are basically all the same, white with the red stripe, similar controls to the tube bombardier 01800 CRRC 01900 for orange 01200,01300 series trash cans CRRC 1400's, new trains that are also garbage blue lines only use the 0700's at this time green line uses type 7, 3600-3700 type 8 3800 type 9 3900 we probably wont ever get all those but you asked what runs so there ya go
