Toe rails are recommended by OSHA on pedestrian bridges.
But seeing the previous pics, they put it in for structural reasons as Jason & Spence mentioned.
The cap board on top is also required when the top guardrail is not made of 2x material.
Boardwalks should have a curb if going off of it poses a significant hazard (deep water?)
From a mtb perspective, i'd say a middle rail is more important to prevent a wheel from going under the top rail,
or hitting a post.
See bridge which
@jklett & I worked on.