Use sweep for this - select the circle, then hit Construct / Sweep, select the path curve then hit Done or Right-click in a viewport (another way to indicate "Done").

If the circle is outside of the bounding box of the rail curve, it will be automatically rotated and aligned to the start of the path curve. If the circle is within the bounding box of the rail curve, it will be used in its current location and then swept along the path.

The auto-place mode is nice, because you can just draw the circle right flat on the world plane, you don't have to worry about getting it in the right location. You can also have multiple cross-sections auto placed, just arrange them left to right. Let me know if this doesn't make sense.

- Michael