Concave Cabinet Scraper!
Yup, this is my current favorite tool. These simple, tiny, thin as a credit card little scrapers are what I use to create consistent rounded rails. Most folks use a router at this point, but I don’t own a router and well, I am sort of in this hand-tools phase.
Once the boards are cut out, the edges/rails are sharp and a bit uneven. I do the rough clean up with my surform to take out the unevenness. Now, sand to further smooth things out (it’s best to sand before using the scraper, they will stay sharp longer)…so, sand smooth, maybe even take a little of the sharp edge off.
Now, turn the board up on its side, clamp in down securely to the workbench. Start with the largest concave and begin running it along the length of the board. I gradually progress down in size until I get down to the most snug fit (without gouging into the board). Now, sand with a fine grit sandpaper.
Voila, rounded rails!