Autoplace just takes a profile curve that's flat on the cplane and moves and rotates it so that it's perpendicular to the start tangent of the rail. Then after that it's the same thing as doing a sweep with just one profile at the start of the rail.

Then the main thing that will govern a long sweep that uses just 1 profile is the twist behavior for how the profile rotates as it travels along the rail.

The default freeform twist gradually banks the profile as it travels along the path. If you set twist = flat you can then set a direction and the profile will be constrained to only rotate around that particular direction as it travels along the path instead of banking more freely.

If neither of those gives you the kind of banking behavior that you need, then you may need to fill in a bunch of profiles like this to more specifically control it. I guess in this case you would want a similar kind of constraint as the twist=flat but instead of it being in just one constant direction it would be constrained to the tangent of the centerline curve.

- Michael