A part of this involves how the deck of that bridge is structured. If the tie ends can be exposed, or there is only a slight flange at the ends, you can use periodic switch ties, as if building for third rail, and carry the walkway flooring and railing-stanchion bases on them. Adze their undersides and then coat the cuts with creosote if needed to fit.
Width is 'whatever your switch tie length permits' and railing height is probably about 4', adequate height that a slip while toting something won't flip a worker over it or slide under it -- this is why at least one additional lower rail is provided. Alternate the direction of the long ties to have a walkway both sides. If in 'dry' weather or with steam locomotives, consider putting water barrels every so often to deal with tie fires...
Older walkways were wood boards, with ends probably butted over tie centers and spiked. Expanded metal drains better, will not rot unexpectedly, and gives much better traction in wet or icy weather, but will have to be watched for rust or periodically painted (perhaps when the steel trusses are inspected).
You could use brackets from the truss structure to hold the walkway entirely free from the track structure, but that would likely be more expensive (and harder to implement) in real life.
The bridge pictured uses a variant of this construction but has a curious way of attaching the railing to the tie extensions. I think I'd have used sockets in the top rather than brackets cross-screwed toward the bottom; there is probably some shear relief I don't know about in there.