Sweep profile along a compound surface?

I’m trying to make a half-round profile follow a compound surface. Sweep1 twists it. I can’t figure out how to get a second rail at the uniform correct distance to use Sweep 2. Thoughts?
BARREL.3dm (433.2 KB)

Pipe around the first rail with radius equal to the desired distance between rails
Intersect pipe with surface to create second rail

Note: OffsetCrvOnSrf does not work for this purpose because the offset distance is along the surface, not in a straight line.

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