The Bikeway Selection Guide from the Federal Highway Administration will help planners make informed decisions about the selection of bike- way types. This guide emphasizes engineering judgment, design flexibility, documentation, and experimentation.
The best answer that you will get for how wide a trail should be is “It depends.”
Survey of skills and competencies to assist in developing a national training strategy for National Scenic and Historic Trails
Trailshaping is a system of understanding in which simple, everyday forces shape (generate) the big picture, details, and nuances of all trails and all trail types, as well as context-specific trail planning, design, construction, maintenance, and management.
One of the most difficult trail facilities to accomplish is a crossing of an active rail line.