There are several ways how "allign" the model you are using to follow the path.
1) After distribution with Creation Tools you can rotate them "manually" in Transform tab (by putting the angle corresponding to the height difference). Please check attached screenshots.
2) You can create a Surface following the spline (for example using centred Outline function in Edit Spline, then Edit Poly modifier and map it by face using UVW map modifier). Then by creating new Forest object based on this surface distribute the object over it using UV Mode. In this case by putting 0 value in Direction the object will be always alligned to it's normal. Then by adjusting Density in Distribution Map tweak the final look.
Personally I'll prefer the second option as if the curve is changing the angle the geometry will maintain the normal direction.
Hope it helps you. By the way in this case you'll archive better results using very simple definition with RailClone. Feel free to do some quick test with Lite version. Thanks in advance.