Thanks for posting. This is an interesting project and while I think RailClone could certainly be helpful there may be a few limitations in this case. I'll address each question separately.
But as I understand RailClone can only use one splines, yes?
Linear generators allow you to use a single spline as a path, Array generators allow you to use 2, one to define the X axis and one for the Y. You could attach splines together for use in a single generator or alternatively there is nothing to stop you from adding as many generators to a style as you need, each with their own spline.
Do you have a suggestion for how I can control the banking and rolling of the tracks?
Although RailClone will bank the segments to follow the spline and you can globally rotate all the segments together, at the moment there isn't a way to adjust the banking at specific points along the the path and have the segments smoothly interpolate between them.
Another question on the same topic, I want to create supports for the rollercoaster in wood. I suppose this could be done as well with RailsClone?
There are two ways you might want to do this. Either create supports whose top follows the path and the bottom is simply intersects the ground or alternatively you could create a style that scales the supports to fit the space between the track and the Z Position of the RailClone object's pivot point. We will be releasing a bridge tutorial in the near future that uses this technique but it very much depends on the construction and complexity of the framework you wish to create. In your pictures I notice the supports follows an uneven ground plane, RailClone does not at present support the ability to scale between two splines, so this aspect would be difficult to recreate.
I hope this helps. If there's anything more I can help with or you have more specific questions and/or scene files I can take a look at, please let me know.