This should be in FPS Model section. Anyway I will give you some tips about stair segment.
1 - avoid having the stair as a single part/mesh: try to make every step a part, may be the same step mesh (*.x file) in different parts with different y,z offsets; the same is true for railings, etc.
this is important for collision and avoiding player be stuck
2 - segment visibility in *.fps file
;Segment Visibility
visoverlay = 1
visfloor = -1
visroof = -1
viswallb = -1
viswallr = -1
viswallf = -1
viswalll = -1
viscornertl = -1
viscornertr = -1
viscornerbr = -1
viscornerbl = -1
