Hello Barry! Really a incredible idea for a railroad diorama and good composition of it!

In my opinion, for better result of your work, I suggest you to perform weathering on ballast stone since they're too flat appeared. I already try such work, but I'm not sure that I'm right. Since I think the gradation of its' color was satisfied for me, I can show the example:

The image shown below was the comparison between previous and after work of weathering performed:

I'm sure that you can do better than me mate! ^^ Once again, congratulations for your finished diorama, I also learn a lot, and many thanks for sharing

Warm regards from Indonesia

Garry