Ha!  Didn't think about it that way.

"W" presumable stands for West, which makes sense.  I think the "G" is supposed to stand for Gold Line, but that's a waste as there's no clear connection to anything in that direction.

The A-line fits for Airport.  N-line fits for North.  The B-line fits for Boulder (well, 30 years from now anyway... until then it's just another reminder of RTD's incompetence).

I guess the "R" is supposed to stand for Aurora, since the "A" was already taken??

Then it looks like they just gave up trying to match anything up with C, D, E, F, and H.