Some nits to pick... Railroad roadway not maintained for 17 years....
Modern rail roadways require large amounts of technical maintenance today by machines/electronics. Using a rail roadway that had not been maintained for 17 years, would mean derailment just when it would happen. Also the bridges also require the same as rail roadways.
Where they did get that locomotive with out using a rail roadway..not something you can get 100 men to pull along like a Helicopter in the woods.
Then the logistics for the locomotive ie fuel(wood) and water.
As for the Monro Army becoming Gandy Dancers to maintain the rail roadway...since there are currently no Gandy Dancers to instruct them on the "correct" way to do the work...They would have to learn the "hard way". that would mean lots of train wrecks, before they get a clue.
Yes long ago train logistics ie fuel and water, but those supply way stations are long gone before the power went away.