When bending Atlas 3 rail flex I stagger the rail joints 1/3 the length of the 40" track. Works out to be in the neighborhood of 13 +/- inches. This prevents the peaking or slight angle which can occur with the overly stiff Atlas rail joints.
Where the uncut rail ends come together I use a rotary tool to relieve the plastic tie strip for the rail joiner.
This shot is of my recent valley line addition which is 20" off the floor. Note how smooth the curvature is. Where every rail joint is there are two rails smoothly maintaining the radius.
The Gargraves bends easily to almost any radius. Cut off the first inch or two and you have a smooth right of way. Here are two lengths of Gargraves in a consistent 16" radius circle:
Even old wood tie GG can flex to provide nice "S" curves"