on Mon Oct 21 20:00:29 2019, in response to Re: Subway route letter uses, posted by Elkeeper on Mon Oct 21 11:00:55 2019.
Why? The single/double letter system made perfect sense. trains that operate express anywhere along their route were given single letters and trains that were pure local with no express running anywhere at all were given double letters. Single letter lines that ran lcl during off hours would carry the double letter of that line. For example the midnight A service running lcl would be AA and the midnight N service running lcl all the way would be a NN etc.