Well, if someone gave me free capital and I could invest it in different ways, I think I would care a lot about how many customers each option created.....sure, I would also care about operating costs and the net benefit each of those new customers brought me.....but if one option did not produce any new customers (extreme example) it makes the calc very easy
Creating a new line that adds capacity to the network where it needs it most is the most efficient way of generating ridership. The upsteam portions of the system that are not yet at capacity cannot reach their full potential until the areas downstream have enough capacity to be able to handle the increased load. Expanding the suburban system means very little in terms of overall system capacity benefits if the capacity of the network leading into a single point (or area) are congested and unable to handle the increased demand. The overall capacity of a linear system is determined by the most congested point. Increase the capacity at the most congested point, and the entire system will flow more efficiently. (And yes, I know that the subway system is not a linear system, but in the peak hour, it functions very much like a linear system. Morning = in, afternoon = out).