For some time, I kept passing by the closed-off train tunnel to the west of Ranger Station Charlie (it's roughly between Harper's Shack and RS Charlie), but I could never find a way into it. I saw a marker near it, but to my surprise, it turned out to be a deathclaw nest (Dead Wind Cavern).

(Good thing I checked the Fallout Wiki before I went in there. Yikes.)

I suspect it's a train shelter of some sort, since it doesn't connect up with any rail lines. Does anyone have news to the contrary?