A wooden railroad bridge, or trestle, is an impressive sight on a model railroad, largely because it is complex and dramatic, which means viewers can instantly appreciate the work involved. Prefabricated bridge kits are fairly easy to find and modify if necessary, but if you want to build your own trestle, it is possible to do with basswood, basic modeling tools and the desire to see the project completed. Each project will differ according to scale and landscape, so you will need to start with measurement and planning.
Measure the length and height of the area that the bridge will span to determine the number of supports you will need and the amount of wood necessary.
Draw out a pattern for each supporting structure. The two most common types are a modified A frame with a top wide enough to support the train tracks or an extended brace that looks like a Roman numeral III with crossing X supports and a top and bottom brace.
Build support structures individually by cutting uniform beams from basswood to match the plan. Use a razor saw or hack saw for precision and control.
Assemble the supports by gluing the joints together with wood glue and hold them in place with clamps. Thin finishing nails might be useful to add strength if the structures are large enough to take a little hammering.
Insert the support structures into the scenery beginning at one end of the layout and working to the other, making sure the distance between each is uniform and to scale.
Lay in the track beams in sections as you insert the bridge supports to hold the frames in place, securing with wood glue.
Allow the completed structure to cure at least 24 hours before proceeding.
Place your track sections on the completed bridge, connect the rails and test the track.
Cutting and assembling the rail supports will be easier if you create a jig, or pattern, that makes the elements uniform.
If you are building the trestle for a garden layout, weather-resistant wood like cedar is best.
Don't confuse basswood and balsa wood. Both are light and have tiny grain patterns, but balsa is much softer and is unlikely to reliably support anything heavier than N scale.