Hi,

I don't understand where is your problem, but could avoid this problem.

You could extrude your profiles to have a straight version then you flow it to do what you want using your rail as flow curve.

First you do your union to have profiles
Extrude it (exemple 10mm)
do a line perpendicular 10mm.
Flow (with stretch) your 2 solids (the extrude solids) with the 10mm line as first curve and your curve (ex-rail) as second line.