Despite my instant reaction being definitely 'railings', based on dozens of visits the RhB over 60 years - this question made me doubt myself!
So I had to go back to lots of photos of RhB to double check. I would definitely add railings. Even if not the modern version, all pictures of my own and published in books show railings on all but the shortest bridge.
I have seen a few short 'culvert' bridges and retaining walls apparently without railings but it looks as though the rule might be that railings are required on anything longer than a single arch structure.
My 2010 pictures show no railings on the small bridge on the lower loop at Berguen, but railings on the small bridge on the middle level.
To add additional debate - I find the size of the stone blocks in your small viaduct too large. Looking at RhB stone infrastructure, IMO each stone block should be about the size of a headlight on an RhB loco