Here is a video that shows how I build wooden trestles from cedar. Most of the included images are fairly self-explanatory. I start by cutting all of the required lumber from cedar fence boards (not shown), and drawing a plan for the trestle components. I make jigs for the bents so that I can cut the bottoms evenly on my table saw. I glue and nail the pieces together, and spike the rails to the ties. The tie plates and NBW castings are optional and do not make the trestle any stronger; I just think it looks better this way.
Bob
[youtube]http://www.youtube.com/watch?feature=player_detailpage&v=OkMeQFDif28[/youtube]