The speaker drives in a modest car beneath an old iron trellis, a structure that carries trains on a schedule so long-running that the times seem worn into the broken ties. Passing under it, the speaker begins thinking about how small and bound human lives are. The mountains don't care about our social customs, yet we obey them from the first day on the road until an unavoidable dusk.
The mood is quiet, wry, and a little fatalistic. The image of dust is the heart of it: we all end up in the dust of our fathers, and ours mixes with theirs. Then the poem turns almost comic. The trellis carries a bit of graffiti, "JEFF WAS HERE," and the speaker says "we have dutifully followed." That line suggests that people trudge along the same routes as everyone before them, and that even a casual scrawl can feel like a signpost. The tone mixes humility with gentle irony.