^^ The main reason why Intamin have changed the track system is that this new double spine track is even stiffer then the quad-rail track which means that they can reduce the number of supports on hills/lifts/etc, meaning that these new large coasters use less materials and thus becomes cheaper in the end. I have a feeling that this new track might even be easier to manufacture then to make all the small parts that a quad-rail track have.
Reducing the cost and ease of manufacturing is probably also the reasons why they have started to use square tubing/I-beams on the supports then large pipes, square tubing/I-beams are a lot more common as a construction material and thus cheaper to percuase...
If you look on the track for the Abu Dhabi launcher then there is grades of strength on this new track as well, (like the bi-rail, tri-rail and quad-rail), this new system have: Double spine, Single spine and bi-rail track.