As I wrote a couple of weeks ago, there are times when building new transit is not warranted. In particular, it is a waste of resources to build new radial capacity on the outskirts if it connects to a trunk line that can't absorb that capacity. I looked at the train crossings into Manhattan to see which could add trains in the peak. That information can suggest where to add branch lines. But what about extending existing lines or encouraging infill development?
The NYMTC's Hub Bound Travel Study has some useful data, but first it's important to give some context. They have an estimate for space per passenger, but it's only broken down by sector, not by line. They do have counts of train cars and passengers by line, so we can get passengers per car. The problem with that is that the cars are different sizes, so there is a lot more free space in an R train car with 341 people than a car with 308 people on the #4 train.
I don't know exactly which equipment was used on which lines in 2012, so I used the most recent train models I knew of. I couldn't find floor area figures, so I used the listed capacities. Please feel free to consult the spreadsheet and suggest revisions. (Edit, April 27: this chart is in fact inaccurate. Please see the comments below for threestationsquare's helpful observations.)
Here is an updated chart based on Table 20:
There are some additional questions that come to mind looking at the chart. If the demand for a line is low, is that always because it doesn't serve enough homes on the outskirts? Maybe, like the J train, it doesn't serve enough jobs. Maybe, like the R train, it's much slower than the alternative. Would extending these lines really get people out of their cars?
There is another question: yeah, an A train car with 713 people on it is really uncomfortable, but isn't a B train car with 429 people pretty uncomfortable too? This is why we need to continue to think about how we can offer comfortable alternatives to wasteful, dangerous individual car trips.