After doing some research, I've started to draw out scale plans for the trestle bridge that will enable the logging branch line to cross a 250 foot span over the mainlines in the area of "Shallow Canyon". This bridge will be entirely scratch built using bass wood with modified Micro Trains flex track incorporated into the bridge's deck.
This view places a draft printout of a dual-level bent arrangement which extends to the full height of the bridge coming in around 70 feet. I'm basing my plans on research into building practices of both the Great Northern and Western Oregon railroads back around the turn of the 20th century. Blending the designs resulted in some basic dimensions with the major cross braces being nominally spaced at 11 foot centers (vertically) and with the vertical bents being nominally spaced at 14 foot centers. There will be something in the range of 18 - 20 bents when it is fully planned out, at least half of those being at the full extended height and then reducing in height at each end where the canyon walls are breached.
At this point, I'm searching out materials for the bridge and finding that it is difficult to find exactly what is needed to be truly on scale with 12" x 12" beams and such. While I can get materials that are somewhat in the ball park, typically running just a bit larger than the desired dimensions, I'll have to rely on a bit of sanding and the use of painting techniques to present the appearance of a suitably scaled bridge. Darker staining treatments will tend to make the materials appear smaller/thinner than they really are.
As I look at the individual materials, it seems that the bridge components will be somewhat delicate but I'm expecting that at it all comes together, the overall project will be much more robust than one might expect. I'll share more details on plans and construction of the bridge as it develops.