When laying track, there's the advanced options pull-out tab located at the bottom.
In that tab, there are options to add, remove, raise splines, among other things. Located at the very bottom on the left is a get track height and apply track height. In the space next to the apply track height, you can type in a numeric value. You then apply that height to each spline point. Once you do that, the spline points will turn yellow indicating that they are now at a fixed height.