I would disagree. Assuming that the bridge in its entirety wasn't damaged, it would be possible to simply reverse engineer the remaining components to make modern replacements. Pin connected truss bridges where popular around the turn of that century because they could be built and test fit at the factory, disassembled and shipped to the site. There, the bridges could be fairly quickly reassembled as everything just kind of pinned together.

The disadvantage of the pin-connected design was that each component was largely critical to the bridge's strength, each component being under either compression or tension. These were susceptible to damage (like in a derailment) that could lead to the failure of the entire bridge. The replacement truss designs had all the components riveted together on site, and these are less susceptible to catastrophic failure when damaged.

Remember that the bridges of this era were over designed, and it really isn't that uncommon to find century old railroad bridges still in service despite carrying loads 4 times what they originally did. And it wasn't uncommon to disassemble bridges and reuse them sometimes at a different length at another location.

Not yet knowing whether UP intends to repair or replace the bridge, I would surmise that the major issue with repair is trying to get an engineer to sign it off, most nowadays seemed to not know legacy engineering and it is just easier to declare it obsolete to eliminate any liability. UP may replace the bridge because it is structurally necessary, or it is the most economical solution. I'm not sure a pre-stressed concrete span is appropriate for this location. I wouldn't be surprised if they don't repair the bridge (even temporarily) or install a modern welded truss replacement.