I did a bit of colorizing of the plaster. This was done using the Woodland Scenics earth undercoat pigment mixed with some water. The effect seems be a good base color although I'm finding that I have to be careful not to over do it with the water since it seems to soften the plaster a bit too much.
All of the track paint work is complete with ties colored to a brown tone and rails rusted. Once I get all of the plaster work that comes next to the tracks completed with color, then I proceed to the ballasting step. I plan to add appropriate weeds and bushes in and around the tracks as part of this step. I may also try to add a bit more rust on the ballast rock closest to the rails to model the same effect that I've seen in real settings where rail rust flakes off leaving stains.
One of the next steps is to setup the removable based alignment system, probably a series of pegs in the upper pieces which will align with holes in the lower bases so that the final scenery will all line up and the tops will be stabilized somewhat.