Question: Do you know what the organizational principle is for layering subway lines on the current NYC map?

I received a question from meltedchandelier​ yesterday that asked:

Hi, I’m hoping you can solve a query that a friend and I have been able to figure out forever. Do you know what the organizational principle is for layering certain NYC subway lines on top of others where they intersect on the current version of the map is? It’s not just BMT/IRT (I don’t think) and it’s not alphabetical/numerical. We’ve been trying to solve it forever, but can’t come up with a solution that satisfies all the intersections.

Now, I can’t definitively answer this question – as I’m not the designer – but I would hazard a guess that it’s simply based on what makes the route lines easier to follow, rather than any BMT/IRT/number/letter formula. This is the approach that I use myself when designing transit maps – it’s more important for the user to be able to trace their route easily, rather than trying to replicate the actual physical placement of intersecting lines. Once you’re in the subway, does it really matter that your tunnel passes above or below another tunnel? Not really: you just want to know what the next station is.

While examining the subway map while pondering this question, I did come across one rather nonsensical arrangement of lines that I feel really should and could be depicted more accurately. As seen above, the “D” and “N” lines interchange in Brooklyn at the New Utrecht Avenue/62nd Street complex. In real life, the “N” is in a trench that runs below the elevated “D”, and New Utrecht Avenue station is located to the south-east of the 62nd Street station. 

However, on the subway map (rotated to put north at the top of the image), the “N” inexplicably crosses above the “D” and New Utrecht station is shown to the north-west of 62nd Street. I suspect the odd placement of the station is because of its label, which wouldn’t fit very neatly on the other side of the avenue (clashing with other labels, etc.). However, there’s no reason at all why the two route lines can’t be placed in their correct stacking order, with the “D” sitting properly above the “N”. In this location, there’s only one route line crossing one other, so there’s no design advantage to be gained by stacking them incorrectly for the purposes of legibility.