Derailed,
Regarding a safety railing, were it I, I would landscape in front of the garden blocks to form a small ridge that would simulate a cut. This would form your barrier to keep things from rolling off when the derailment eventually happens at that curve. We have also found landscaping from the raised bed to our yard is visually pleasing, as it helps to transition the eye and mind from the 1:1 world to the 1:24-ish world. In our case, we dug a dry gulch to justify some bridges. I don’t have a good picture of how it looks now, but here it is under construction several years ago to illustrate my point:

I can find or take a new picture if desired. In your case, the process would be reversed, building up from ground level, a project we are considering in our “pinch point” where we need to broaden the curves. If those curves bring the rails too close to the edge, we will build up a cliff from ground level. That will bring the curves as close the corner as we can and provide a base to extend the existing protective “cut.”
If you are handier or more artistic than I, you could also consider a viaduct, building or painting the viaduct onto the retaining wall then building its deck, complete with safety rails, on the top. It would provide the same visual transition from 1:1 to 1:[your scale] and the extra safety margin. We have contemplated a similar idea at another place that could be better served with broader curves. For now, it is a “project too far.”
Eric
Edited so grammar it looks like.