As a regular user of the MBTA during rush hour commuting I feel obliged to remark that full capacity on a subway train is neither a comfortable or sustainable condition. Any transportation system that runs at full capacity represents an unusual and probably undesirable situation. Private automobiles have seats for four people but are mostly used for single person trips. To assign a car an average capacity utilization of 25% is actually generous because it only accounts for the physical space occupied by a user. If we factor in usage rates on an hourly basis, automobiles are hardly used at all--most of their time is spent in a parking space, patiently waiting for the next fifteen minute trip to and from the Shop-o-Mart.
I don't know if there is a capacity utilization "sweet spot" for any transportation network. Attempts to quantify such a phenomenon would lead to qualitative criticisms of every assumption made about terms such as "capacity utilization rate" or "line haul capacity." It makes for an interesting calculus problem that I would never be able to solve. However,the MBTA has made considerable investments in a far-flung commuter rail system which operates well below capacity for several lines. Even with the anticipation of future growth, higher density areas subsidize lower density transit networks with the net result that all systems are starved for resources that maintain and improve quality of service.
As a though experiment, I wonder how a city like Boston would be able to function in the absence of the MBTA?
Another interesting component of the discussion to consider is, how much of our notion of "full capacity" is cultural? On the Tokyo subways, there are employees whose job it is to stuff people onto the train until no more can fit. Their trains' "capacity" to hold riders is higher than ours...mainly because they are more accepting of having strangers shove them into other strangers' faces. If we tried to institute that system in America, no one would put up with it. We have a larger "personal space bubble," and we want our designed environment to accomodate that. On public transit, we will allow certain intrusions into our bubble...but only to an extent.
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