The USDA Forest Service Standard Trail Plans and Specifications is a comprehensive guide to the design, construction, and maintenance of National Forest System trails and trail bridges.
This is very specific information that shows how to develop a trail plan, plan out the needed materials and labor costs, and then actually build each part of the project.
When it comes time for actual trail project planning and implementation, the resources here are invaluable. There are individual drawings (autocad) available here for everything from the trail bed to complex bridges and signs.
Trail Cross Section
Switchback with Circular Landing
PLEASE NOTE: Much of the above has been directly copied from referenced sources publicly available on the internet. Subjective opinion and interpretation can be found in color-hilited text boxes.