The Kappler Mill & Lumber Co is a supplier of scale lumber that is often used for building structures for model train layouts.
Kappler Mill & Lumber Co is offering several free tutorials for projects. Most of them seem to be for the HO scale, but most of the tutorials offer dimensions that could be scaled in other model train scales. Some of the tutorials are more detailed than others and there are tutorials for structures and rail cars.
I was drawn in by their Building a Footbridge and Small Wooden Freight Station articles. These projects look like they would fun to build and be easy to assemble.