If I understand correctly, you have tried putting train track models in the air, so that players can see the bottom of them? Well, quite simply, they don't have a bottom (probably just to cut down on the file size, since you aren't supposed to see the bottom).
Train tracks aren't supposed to be suspended in the air. Even if you could see their bottoms, it would look ridiculous. Have you ever seen a train run over train tracks that float in midair? You have to put them on something, like a bridge.