Have you ever spend hours on a model only to find it doesn't really fit in the scene the way you thought it would? Before you commit a substantial part of your life to a scratch built structure its a good idea to do
a mock up. You know, cut out some cardboard, figure out how high you want that structure to be, check the depth, maybe draw some loading doors and windows on it to see how those 40' boxcars line up. We'll if your going to build with wood or styrene, the next step would be to take those mock up patterns and transfer measurements onto the styrene or wood. On the other hand, if you use the black foam core for mock ups the patterns get used and you're already well on the way to having your structure.
This mock up showed that I was hiding too much of the blue factory that extends over trackwork and some brick arches that I really wanted to show.