So I started out lining my garage attic with boards. They are 1.5 ft above ground. I used attic flooring from wickes as the attic is very very dry and they were cheap. Once the basic frames and boards were put up I lined it with some track. At the moment I only have 2 loops and some sidings. I'm still working on my track plan. 

Where the line goes into the arch sections of the roof I'll make a removable tunnel.