You are kidding. They use (small) wooden trestle bridges TODAY. Cheap, readily available materials, easy to repair. Railway lines are constantly maintained anyway, so its not a stretch to keep a wooden bridge up as well.
Scroll to bottom-ish of this for some big wood bridge examples: http://www.highestbridges.com/wiki/index.php%3Ftitle%3DA_Brief_History_of_High_Railway_Bridges
Wooden trestle bridges require no special skills to build, and are of cheap materials (sourced on site often). The early US, and later on as we moved out west, had long distances so limited resources of the stone and steel type, and a lot more unskilled labor than skilled. At the least, we're often into speed, so a 2 month job putting up a big wooden bridge is better than a 2 year iron span. Later they get replaced.
https://www.archives.gov/publications/prologue/2011/summer/usmrr.html
Among Haupt's most challenging assignments was restoring the Richmond, Fredericksburg, and Potomac Railroad mainline from Alexandria to Aquia Creek and Fredricksburg.
One of his most famous accomplishments, the one that caught Lincoln's eye, was the reconstruction of the Potomac Creek Bridge, which he repaired in nine days with (along with cornstalks and beanpoles) an inadequate supply of tools, inexperienced help, and in rainy weather. After completion, the bridge carried 10 to 20 trains a day.
Note, days to build with "unskilled infantry."
Not even slightly a plot hole, not even an improbability, but an almost certainty that they'd have mostly or exclusively wooden bridges on this line in this time.
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