This is how one can get smooth and strong joints along flex track curves:
Note: you can pre- curve flex by carefully bending it, and this really helps if you partially bend the sliding rail at its protruding end first to somewhat conform to the intended curve radius.
You lay the first curve length and use track nails for a temporary hold. In foam use twice as many, even using a thin drill bit to make some extra holes. The sliding inner rail should now protrude.
Take the next length and slide the protruding inner rail into the spike heads of the new piece. Lay the new piece flat and along the curve, and mark where it abuts up to the inner recessed rail of the new length. Remove the new piece and remove no fewer than two plastic ties under that mark, three is probably better for tighter radii like 18”. Then reinstall the new piece.
As you slide the two together, insert a metal joiner in the tieless gap so that both rail ends meet...BUT...not up tight. You want to leave about one mm gap for rail travel due to any heat or humidity factors that change over seasons.
Later, you use a needle file to file away the spikehead details and the plate details on those removed ties so that when you slide them under the joiner they won’t cause a local bump at the joint.