The SFMTA published a blog post explaining just how the N-Judah, J-Church, L-Taraval and other lines, both defunct and contemporary, got their letters. Turns out the answers are far more straightforward than today's seemingly random letters might indicate. With the exception of the more modern T-Third Street and E-Embarcadero, the lines were named alphabetically from A through O, but only five of the 16 older lines remain. Sadly, you won't see an A-Geary/Park, E-Union, or O-Van Ness on city streets.
Read more about it here.
