Railroad "Civil" engineers are the most , um, conservative there are.
Part of that is their observation of the effects of neglect on bridge strength- and there has been lots of that- called "deferred maintenance"
But there is also a lot of Ludditeness at work. Why else would Cooper loadings still be used- double headed 2-8-0s ??????? Not that saner alternatives haven't been proposed decades ago.
Also, rational analysis/thinking gives way to more is better, even if the more does absolutely nothing but increase cost.
In my experience with certain publicly operated outfits I have been appalled at the numbbrains who put your tax dollars to waste. I was directed by a certain electrically operated (once known as trolleys, but now lighter)authority to design a bridge for the highest Cooper load PLUS STEAM IMPACT by their engineer- who had previously worked for a western mainline road.
There are a few enlightened souls out there, else there'd be no prestressed concrete bridges and ties, or welded steel bridges. Actuarily speaking, it may get better as Death reaps its magic. The rule is, start working as a bureaucrat and stop thinking, that might keep you awake during the day.
On a more positive note, I would be very surprised if easily dismantled spans DIDN'T go into somebody's "bridge bank". Much steel, particularly heavy structural sections and thick plates are mostly manufactured offshore, and lead times are weeks (recently months), while adaptation and or modification of existing pieces could be accomplished in days. So the old habit of not wasting anything is still valid. As recent example, the "new" Metrolink bridges in Soledad Canyon were assembled from "used" stock. And I should mention that Mike McGinly (spell?) who concocted that and other Metrolink projects has been the shining exception to the rule I made up, above.