Well now. Whilst preparing to trim the bathroom window in my addition I had purchased plastic (styrene I think) trim. While cutting it with my saw I got to thinking that if the stuff was a little bit less wide and didn't have the fancy molded stepped edge. The stuff I have is 3 1/4" wide by 7/16ths thick. If it was available in 2 1/2" by 7/16" it would make a great retaining wall... The length is somewhat flexible and I bet it could be bent to required shape with a heat gun.
Another thing to put on my "when I build a track" list...

Scott