Won't the wheels drop into the frog if there isn't something in place for the wheel flanges to ride on?

No. Wheel drop in the frogs isn't a problem when turnouts or crossings are precisely constructed. As wheel flanges vary in size from one manufacturer to another, it isn't easy to fill in the base of the frog for the flanges to ride on and ensure reliable operation. There will either be clearance under the wheel flange or interference between the bottom of the frog and the flange that the wheel will "bump" when going through the frog, so it is best to leave it empty.

Check out the video to learn more about why Fast Tracks-built turnouts do not have a wheel drop problem.