Have you tried resizing Default segment, instead Marker segment ?
I did a quick test using XSectionLength, and it works fine.
Just it's necessary to attach some Segment to the Marker slot (in my case an empty segment). If not spline is not divided by Markers.
Resizing Marker segment is more tricky, because Markers are added to the start/end of each section, alignment may change, etc.
Internally, RailClone works dividing the spline in sections (by Marker/Envely/Corner), and then processing each section in a Start-Default-End sequence.
Default segment is always the same (Default slot), but Start/End segments depends how section was divided. There are many variables affected, depending of the section.