Why Does Fallout 4's Subway System Go NOWHERE?

Fallout 4 (much like Fallout 3) has a series of subway stations. However, there is a major difference in how the games use these locations.
CHOO CHOO

IRL Boston Videos:
@DCtheABC
https://www.youtube.com/watch?v=3LRPHOsqkXs&t=171s
@Transport_International
https://www.youtube.com/watch?v=d02SM9R6TuU&t=88s
@Thom-TRA
https://www.youtube.com/watch?v=9LtxvsHJpGQ&t=831s
@BostonAreaHomes
https://www.youtube.com/watch?v=wQ-3x6fAKcg&t=127s