I am pretty sure that the rail tool is not clever enough to stretch a custom panel to suit the angle of the stairs.
I am pretty sure you would have to create the custom panel to suit the slope of the stair.
Barry.
One of the forum moderators. Versions 6.5 to 27 i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10 Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11