Work Procedure :
1. Prepare seperated stairs mesh
2. Make stairs joint, stairs rail curve, top linear rail curve with equal spans
3. Align stairs rail curve with equal spans using top linear rail curve and lofted rail surface
4. Make stairs rail movement with expression
5. Make hand rail movement with expression ( pre-curveWarped hand rail model )
3. reference - https://forum.highend3d.com/t/intersect-curve-with-surface/50147