Race Track Question

Are race tracks for racing games usually created as a terrain with a fitted texture, or is the road surface created as a separate mesh?

I assume you mean an asphalt track. In that case it is usually better to model the road as a separate piece from the rest of the ground (or, if you want road and ground as a single mesh, do a custom modeled mesh and not a terrain.) For an off-road track just texture a terrain appropriately.