A failing rail bed is not as uncommon an occurrence as one might think. There are probably thousands more throughout the US in various stages of degradation.
The railroads have been at this since before the Civil War, and still, rails are not foolproof. The roadbed is actually a high-maintenance part of railroading.
And it has to be coarse crushed stone ballast and wooden ties to take the pounding of heavy trains passing over every inch, there can be no substitutes if the traffic is really heavy or relatively fast. There have been stressed-concrete, plastic composite. and even steel ties used, but their use is limited to slow-speed, like a switching yard, or low traffic weight.
Looks like concrete ties.
Think about it: every heavily-laden wheel that passes over any point on any rail is like submitting it to cold-forging or cold-rolling forces.
Sooner or late, some degree of metal fatigue will occur...
Well said.