For this to realistically work, the subway system would have to have a major revamp. As it is now, you just walk down a set of stairs and ‘onto’ the subway line, no platform or anything. IRL subways usually have hub locations, with 2-3 crossing subway lines in a central location; and unless somebody wants to make diagonal subway lines, you could only have NS and EW lines.
It would probably be best to make a new tile (or several) to put next to the subway lines, to indicate a platform with turnstiles, vending machines, chairs, etc.
EDIT: Along with small ‘walkways’ on the edges of the actual subway line, possibly with a few utility/maintenance rooms here and there.
Maybe make a new readable map that reveals the subway lines?