Ship tracking showed 8.5kn when the power failed and slowing to 7.5kn just before impact. There are not many structures that could withstand that kind of direct impact.
Which option makes them calc out to 99.9% stress? Seriously, there was no extra capacity in those old panelized roofs. I probably get called out to develop a fix for a cracked 4x14 once a month here in the Southwest.
Take a look at the particular UL assembly you are using, the reductions should be outlined there. If you are using IBC prescriptive fire requirements, take a look at Table 721.1(2), foot note 'm'.
It looks the actual derailment caused the collapse of the single span and then dragging one of the derailed cars caught the ties on fire further across the bridge. Its a riveted steel bridge constructed in 1912.