The sign post just past the culvert does point to a real trail. It switchbacks up above the railroad tracks, then follows a loosely marked cairn route along the shelf overlooking the canyon. There are times when no cairns are visible at all - you basically just keep following the shelf north until you reach the loop junction sign. Looking down, it's clear that the route on the canyon floor is significantly better defined.
Rendarth is correct about the trail sign and the route on the west shelf. At the loop junction sign the trail splits in two. Taking either branch returns you to this junction, the route going through the opening of the arch. Either direction involves brief scrambling (about six times) where the trail steeply ascends rock steps.
About the route on the canyon floor... I've gone that way two or three times, to reach areas to the northeast. The route is well-defined by the vertical canyon walls, and is interesting for its own characteristics, but it ends at a tall pour-off far short of the arch. I've heard that it's possible to scramble past this pour-off but do not know the route. Going up the canyon bottom would take much longer than does the shelf trail, assuming one can find the way. Plus, while on the canyon bottom, you don't have the sweeping views one has from the shelf trail. firstname.lastname@example.org
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