Inno is right that the feature curves need to exist in the input.
If the input is a Brep, there should be some vertices along a feature curve, and if it is a Brep, there should be a split or seam that matches the feature.
(you don’t need to explode polylines to use them as features though)
The issue here I can see is that where you’ve booleaned away those rail like bits from the base, those patches don’t have a seam where you’re putting the feature curve.
so the mesh generated there won’t stick vertices along the curve there
If you want a mesh seam across that gap, you’d need to also split and rejoin(like shown here) that long surface first.