One of the challenges when designing a layout is that inevitably you will create a problem where there is no adequate commercial source to solve your landscaping problem. For me, there were two 'design challenges' that did not have either an economical or suitability for my design, and these were two longer than average 'bridges' and multiple retaining walls.
Overpass / Bridge:
The above photo shows my 26 inch, two line, railroad 'overpass'. This was constructed using 1/8 inch 'Sintra'. which is a material I use liberally all over my layout as sort of a 'sub-roadbed' to sit on top of the styrofoam. It give a nice, rigid base for layout track and scenery, and is also ideal for adding in more rigid structural elements (like this bridge).
The sides of the bridge are made of styrene, while the supports are actually repurposed and cut MTH Steel Elevated Supports that I had leftover from my O Gauge layout, cut in half (they are about 6" for O Scale, and I cut and sanded them down to the 2" I needed). Will complete all the details and painting once the track is in place.
The retaining walls are pretty straight foward. These are just sheets of Evergreen Scale Modules O Scale brick walls, that look like decent retaining walls in N Scale. I then add strips of styrene every two inches or so, carefully placed to cover the seams, to give the effect of the vertical supports and all to the overall effect of a 'real wall'.