Lance, thank you very much for your advice. You are right, the ballast and the soils will be very important. Unfortunately most of the materials you mentioned are not easily available for me here in Germany. If you can’t order it from Walthers, it is getting difficult. So I am trying to get the right visual appearance with the materials I can get here. I already have HO and N scale light gray granite ballast that, judging by the prototype pictures, seems to come close and fine, very lightly colored sand that will give me a good starting point for the ground cover. I still need something like the concrete mix or asphalt mix you mentioned to give it a more dirty look.
Thank you for your kind words about my hydrocal structures. For the structures on my two removable extensions I wanted to use styrene, because I wanted to fix them permanently to make attaching and removing of the extensions as simple as possible. Another reason to use styrene, is to keep the weight of the extensions low. But you are right, the hydrocal structures look more real than most styrene structures. I haven’t started to build any of the structures for the extensions yet, so it is still time to think about it again.