Those regulations were only referring to regular open track. Bridges receive special periodic inspections. They have much better ties and individual bents receive shims to keep the vertical alignment smooth. A good bridge's rigid structure makes it normally the most solid piece of track in the area. The burned trestle was many of several on the CW that had bad "dumps", where the rigid bridge structure meets the comparatively spongy normal roadbed. This pounding results in a vertical drop of up to a couple/three inches within up to 20-30 feet--making a rough transition and ride. This ends up permanently bending the rails to where even a good surface tamp can't make them straighten nicely. Severe bridge dumps can transfer the pumping of the rail back onto the bridge to where the first couple of bridge ties can begin to show crushing under the tie plates, since they can't pump like regular-roadbed ties can. A derailment on a bridge is about the last thing that you want since bridge ties are much bigger (and more expensive) and bridge timbers aren't exactly in stock at most maintenance yards--not to mention the mess around a bridge derailment. These are some of the reasons why older open deck bridges are being replaced with ballast deck "pan" spans wherever possible. Even the BNSF's "long bridge" over Lake Pend Oreille at Sandpoint is currently getting its first phase of a multi-year changeout.