Nicely done. Let me add my 2c:
- the trip to the departure station
- the trip itself on a mass transportation system
- the trip from the arrival station.
Let's assume that the tolerable trip to/from a station is 15 minutes. If you walk, it means that it is approximately 1 mile long. Now imagine a 1-mile sphere around your station. This is a service zone. You better have a lot of people living/working/visiting here. You need skyscrapers to utilize this volume efficiently. Some underground facilities (like underground malls) will be helpful too. How many U.S. cities fit the picture? New York --- yes, Los Angeles --- yes, Chicago --- probably, Houston --- unlikely, and so on (I covered the top 4 most populous cities). My point is: a mass transit system will be unprofitable in almost all American cities or it would be inconvenient to use.
A case in point: it takes me >15 minutes to drive to the nearest DART station, which will take me to the downtown. It takes me 20-25 minutes to drive to the downtown in my car. Could you guess what I do? My DART line goes along 75, which has a speed limit of 60 MPH. DART cannot exceed 45 MPH and stops frequently. It takes more than 20 minutes to get to the downtown using DART. Like everybody else I pay my taxes to support DART (1% of all sales goes there) and like majority of locals I cannot use it.