Mick - The track isn't fixed, it's splines made up from different length sections. The various section lengths make for smoother curves, according to their radii. For example, a straight length of line leading to a curve would be made of 16m leading to 8m, 4m or 2m, depending on the radius of the curve. This is both cosmetic and the longer section splines use less computer resources. Try downloading them and having a practice.
Dave