Thanks for contacting us. RailClone works be deforming geometry along one or more splines but cannot (yet!) adjust the splines themselves, so unfortunately it is not able to automatically calculate the catenary curves created by hanging cables. The curve shown in the tutorial and I suspect in your screenshot was created by modelling the cable with the draping already in place but this unfortunately means you can't align the flags to it.
However it is possible to use RailClone with a base spline that includes the curves, so that the cable's meshes are deformed to follow the path's curve, for example:
In this test I used the Labyrinth
plugin to create a series of catenary curves (shown in pink and exaggerated for effect) that follow a standard spline (white). As labyrinth outputs a spline it can easily be used by RailClone to automatically place a pole on each vertex, and deform a cable in between. You may also find this free script
helpful for creating splines that would create a similar effect. Ideally of course we would like to include the ability to add these types of curves directly in RailClone, and it's on our (very large!) wishlist.
I hope that helps, don't hesitate to let me know if you have further questions.