Its been almost a day and no-one has posted a response. Possibly no-one has an answer or no such track exists (or both).
Unfortunately, I have no answer for your question either but I do have some advice. Most creators do not use multiple (double, triple, etc) track splines because they do suffer from some "issues". One of which is that they cannot be straightened with the track straightening tool. I and many others use single track splines laid parallel to each other. If you want perfect spacing between the tracks, such as in a yard, then there are many track spacing tools available that can do the job.
My views and good luck with your 30in or 760mm gauge project.