I have roadways ( railroads ) That I've sucessfully implemented into the game containing geo, fire LODs, with a raised roadway. It can be done, on a flat surface. I gave up trying on a slope.
If you need it to follow the terrain you will have to use a road smoothing script to create a smooth transition from a flat surface to a grade. You will have to make a nearly perfect brake in the road network and create a custom object to link from the flat surface to the angle of your grade.
In the models properties you will have to include placement=slope.
If not done properly, you will end up with a stair step effect from one piece to another.
It's a huge time consuming process, and not worth it in my opinion. They can lay railroad tracks faster in real life.
I'm saving my track pieces for hope someday they can adapt things like railroads, streams, and rivers in upcoming game engine releases.