No one will probably read this . But I thought I'd comment. Right or wrong - I always thought it was about a man who was planning to be a freight hopper, ditching his girl in Frisco and riding to Montana. He hasn't done it yet, because its all in the future tense, except the second verse where it bothering him. There's no mention of the destination but frisco Colorado had a train line that went to Montana and it reminded me of Zen and the Art of Motorcycle Repair which took place in Montana.
He knew it would hurt her, (gonna miss me when I'm gone) and it bothered him (Lord, I could not take my rest) but he knew it would be for the best (The sun will shine in my back door some day - March winds will blow all my troubles away). He wishes he could just get it over with, and jump that freight train and get out in front of the troubles he feels, because jumping that train holds the cure for what ails him, (I wish I was a headlight on a north-bound train - I'd shine my light through the cool Colorado rain). No matter how hard it would be (I'd rather drink muddy water, sleep in a hollow log )its better than staying and being treated wrong (Than stay here in Frisco, be treated like a dog). But the reason the song is so melancholy is because we don't know if he ever got out, or if he stayed. The most fervent line in the song is the part "I wish I was a headlight on a north-bound train" that's always sung much more forcibly because it's what the protagonist wants.