One of the more interesting ways of detecting rail damage, and subsidence in general, is optically detecting noise / distortion in fiber optic cables. An applied case of observables which are the basis for an evaluative (the "signal") being utilized originally to diagnose possible maintenance issues and then going "hey there, wait a sec, there's a different evaluative we can produce from this exhaust and sell".

https://fibersense.com/

http://www.focus-sensors.com/