Re: Since you ask, here is my 2¢

Bill Tindall, E.Tn.
First off, let me just state, the structural part is not what you need to worry about This bridge has to meet structural codes.

One of the things that I try to avoid,is to never have the broad faces of the lumber together not possible to avoid.

Even the best pressure treatment doesn't permeate the resin rich cells of heart pine, Indeed true and the decay we see on the 100+year old timbers is decay of the timber centers before it was learned how to creosote treat these centers.

One of the changes I have made on the new bridges is to treat to UC4B or CCA as allowed. Treated lumber is plantation grown SYP that has no resin centers.

I think you (not me) have gone way overkill on what was needed. DUH!!! The original design seems ready to accommodate passenger trains. Those are 8 x 8 not 6 x 6. And it doesn't matter. They could be 1 x 1" as no vehicle can cross the bridge without tires directly over 26 x 16" stringers. There is no tension load on the deck. The through bolt yields at 76,000 pounds.

Then I'm not sure what the curb board is supposed to do,
Essential to hold the ties in place and provide a place to secure the posts.

have you taken a look at Timberlock screws? I'll deal with rails later

They will drive even without a pilot hole, although I always do when near the ends of boards, Found it unnecessary to use a pilot either at rail ends or even with driving 8" into an oak tie. They are amazing

I don't know if it was necessary to run the deck boards across rather than longitudinal You must not bike across timber bridges. It's essential to run deck boards perpendicular to bike travel.

then decked with the standard 5/4x6 decking running across. 5/4 decking does not last on trail bridges. It is not uncommon to use 3 x 6.

I like to use 4x4 post, Won't meet 200 pound code, especially so after a few years of weathering. Trail bridges are expected to last 40-50 years. Bridge rails are higher than residential rails, putting more force on post at attachment. The 200 pound code force multiplies by the height of the rail. 4 x 6 dry straighter. And the 5 1/2" span makes joining rail end joints easier. With a post spacing of 4', post cost is not an opportunity to save much by going to 4 x 4.

I would also drop those other 4 2x6s of the railing down to 5/4 x 6" I chose not to fight this battle. I have never seen a trail bridge with a 5/4 rail so there must be some reason to avoid it. Probably the reason to use 2" is for longer serviceable life. These bridges are 80% labor and 20% material cost.

I'll deal with other rail issues later.

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