for example, if the ratio of local:express trains is 5:1, the watershed should be more heavily influenced by what's accessible in 40 minutes via local.
this is also in part due to the way people use the subway in NY -- nobody looks at a timetable and says "i'll head down to the station to catch the 6:37 express" like you would in some cities with reliable timetables/schedules. in NY, you just go to the station and wait and hope the train you want shows up next/soon.