An excellent thread. As several have noted, RRs are notoriously easy to repair, and nobody did it faster & better than the N Koreans and Chinese. As for WW II, Germany had dedicated engineer units for the purpose, and SOMEWHERE in my "archives" are some references.
Also as noted, bridges are the favorite choke points of targeters, as Steve Coonts and I described in last year's "Dragon's Jaw," the Vietnam saga of Thanh Hoa Bridge. But the N Viets (with considerable Chinese help) kept supplies flowing with emphasis on the NE railroad.
Two sidebars:
In the 70s a friend of the P-51/47 persuasion (9AF) said "I woke up the other day with a feeling I hadn't had in 30 years. I wanted to go out and bust a locomotive. Not one of those diesels that just die on the track but one of those 40-ton steam monsters that EXPLODE when you hit 'em with eight .50 calibers."
In the 80s I met a former SE-5 pilot, Canadian living here in Arizona. He mentioned bombing bridges in 1918 and I asked "How did you approach the bridge?"
He said (looking askance) "Diagonally along the length. OF COURSE." (Compensates both for range and deflection errors.)
I said that the vile-putrid LBJ and RSM sometimes required attack headings perpendicular to the span, which of course made a long target into a really narrow one.
He asked "WHY?"
I explained that the vile-putrid LBJ and RSM were concerned about chinese & russian persons at either end.
He just shook his head...