on Tue Oct 29 18:47:35 2019, in response to Re: Subway route letter uses, posted by zac on Mon Oct 28 08:28:51 2019.
However, lines that operate on the BMT could use a slightly different logic from lines that are exclusively IND. The A, C, E, F, G operate only on the IND and not on any BMT lines and could be classified as IND and be lettered by using the IND branch to trunk logic. The B, D, M, and R while operating on the IND also operate as constituent BMT lines and can be considered BMT. The rest of the lines, J, L, N, Q, and W operate entirely on the BMT so there is no ambiguity there although some might question how to classify the Q but since it doesn’t share trackage with any other line than could be considered IND, I would keep it in the BMT family. Even if the upper SAS eventually becomes an official part of the IND, Since the Q is a BMT service for most of its length I would consider it BMT.