Based on my observation and research, toe boards are to prevent tools and such from falling. Curbs are required at edges for
ADA - white canes and wheel chairs. Do you have a source that suggests it to prevent someone from slipping through? IIRC and I'll recheck before too long toe boards are not required by the building and fire codes.
Well, I know that I've thanked my lucky stars that there were toe-boards on many narrow, or otherwise ill-designed catwalks. There is a principle in accident litigation that is called 'positive control' or 'positive guidance' that discusses the ability to guide someone or something in the direction you want them to go so they don't go where you don't want them to go. I'd put toe-boards in that
category. I wouldn't spec an elevated
platform without them -- regardless of the rational (tools, people, junk, not junk). The general requirement is that the gap between the bottom of the toe-board and the top of the walking surface is 1/4" or less, but I spec 0" (NO gap) because I've seen too many thin heavy things that could slice or cut or impale people left on floors unsecured and adjacent to the edges elevated platforms.
Better to just close it up.
I've seen toe-boardless lighting catwalks with tech's sitting on the
catwalk floor and their legs hanging over the
edge while they worked on the lights. I'm thinking: A) that's not a good idea, and B) if there was a toe-board retrofitted on there it would reduce the likelihood that they might slide-off if they still chose to work in that position. I don't think that the
catwalk railing decision-makers were considering the scenario of people working with their legs already hanging over the side of the
catwalk when they decided to say toe-boards are just to keep tools and materials in place. I think toe-boards are an integral part of keeping people on a
catwalk.
With a 45" top
rail (
OSHA's new recommendation after realizing that many people's CG is higher than 42") and no toe board, places the gap between the
catwalk floor and the bottom of a mid-rail at 20.6" (assuming 1.9" o.d. pipe), 20" is plenty of space to fall through. If the top
rail is up higher (say to 48"), then the gap just gets larger. The trouble with slipping and falling
under a railing is that
IF you are cognizant enough to realize what is happening
AND you can catch yourself, then you are left in a VERY awkward position with your back arched and rubbing against the side of the
catwalk. This is not a
point of good leverage if you have low upper body strength and/or are overweight. You may hang-on for a little while, but you may end-up loosing your
grip before someone could assist you. 1.9"-2" O.D. pipes are very common in theatres for
hand rails because they are used for lighting battens, but if you look at the
hand railing requirements, 1.25-1.5" O.D. is suggested because it provides a better (stronger)
grip for most people's hands. If you were hanging from a 2" pipe you probably would not have as long of a hang-time as if you were hanging from a 1.25" O.D. pipe. This is one of the reasons that ladder rungs are 3/4-1" O.D. - better
grip!
I found this
catwalk (see attached) in a
theatre - it has a 3" O.D. top
rail that you couldn't grasp if your life depended upon it, no mid-rail, and no toe boards, so if you started to go, there would be no stoppin' it . . . If I had to chose only one improvement, I'd rather have a toe-board here than a mid-rail (assuming 'replace it' wasn't an option).
Or you could have the scenario of the too low
rail (see 2nd picture - it's about the same elevation as the locking
rail ~30") with no mid-rail and no toe-board and a
very slick sealed concrete walking surface . . .
wrong on so many levels (if you look at this picture closely there are many things wrong in the whole scene, not just the railing).
, was off an extension ladder accessing a 
