I haven't retaining wall, but I have made tunnel portals insulating foam. I cut them out, carve the stones or brick, then apply plaster to it. I can stain it to replicate whatever material I want it to be made of.
Marlon
See pictures of the Clinton-Golden Valley RR
If you cut them (I used a Dremel tool and their reinforced wheels) I suggest doing it outside. The resin creates quite a bit of dust and the smell is not heavenly by any means. I painted them using a variety of gray primer and Krylon camouflage khaki paints. One thing I did need to change and that is that the tops and ends are flat where my prototype had a cut stone appearance like the sides. I took thin balsa wood, glued it to the top and ends , battered at it with a wood burning tool (shades of the old E.L. Moore articles) and then sealed the balsa it with acrylic paints to give it a smooth surface and hide the burn marks and then painted the entire wall with the gray and Krylon camouflage khaki spray paints.
Dave Nelson
I've got 3 of these stone wall molds:
http://www.mrscenery.com/catalog.htm
I've done a lot of retaining walls, so this is a very economical way to go. I cast them with Hydrocal. For some, I simply spray them with cheap rattle-can gray primer from Lowe's. On others, I've used Rustoleum speckled sprays. Then I brush them with an India-Ink-in-water wash to bring out the details.
It takes an iron man to play with a toy iron horse.
Do you have any of these walls on your layout?
http://www.walthers.com/exec/productinfo/214-8304
Did you cut them in any way and/or stain them.
I'd like to see pictures how other modelers used them.
JaRRell