On RailWorks Wiki page, unfortunately, misses documentation how to, except skydome, make moving clouds too. I researched and got some results. On the picture you can see clouds in my own arrangement. Download template and make your own variations. Package contains source files of Blender, 3dCanvas and already made sky.IGS and sky.XML files. If you make something better I hope you will share it with the community.
Note: In template there aren't texture like on picture, there are very simple ones that you need to replace with your own ones. So, take your camera and go in cloud hunt
For download visit my blog.
Vlado