Tough thing to do. The biggest problem is not really a steep angle but the transitions from level to up, up to over, over to down and down to level. You could get away with a steep climb if your transitions include some sort of easement.
If you're using the standard pastic bridge support things, you may need to look for something else. You can attach the track to a board, then use the natural flex of the board to create your easement. When I had plastic track, I considered using something like this: Foam Grade
to start the incline and then use the plastic supports or wood to complete. You could even use this to start the incline but add shims under it as you get higher to get a 6% or 8% grade.
Another option, although a bit tricky, is to bend the track itself, by carefully rolling it over a large dowel. I "faked" some extra dune sections with some off-road plastic track doing this, but it is really easy to wreck the track by buckling the metal rails.