Hiya J - thanks for the welcome on a (probably) fleeting visit - frantic at work, dead puter at home! Yes, I suspect I'd do the same as you: perhaps because the phrasal form "~ line" is familiar? eg, "the up line, the down line..."

I think you hit the mark about a phrase, eta. otoh, I have seen it written (drama context) as a single word... but mebbe that's a kind of term of art case.