A skateboarder is balancing on the rail of stairs.
A scooter is balancing on the rail of stairs.
Either the skateboarder or the scooter is balancing on the rail.  Stairs have rails and/or handrails but there is no such thing as a "rail of stairs".
exists e1 x1 x2 x3.(n_rail(x2) & on(e1,x2) & of(x2,x3) & n_stair(x3) & Actor(e1,x1) & v_balance(e1) & n_skateboarder(x1))
exists e1 x1 x2 x3.(n_rail(x2) & on(e1,x2) & of(x2,x3) & n_stair(x3) & Actor(e1,x1) & v_balance(e1) & n_scooter(x1))