The steel used for rails is
hard stuff.You can't cut it with an ordinary saw. Takes a special blade and a large saw.
https://www.youtube.com/watch?v=yoYBFrJDrlII doubt ANY of these recent derailments will be found to be because of a rail "sawed through".
First off, when you cut a rail like that, you break the circuit that governs the signals, which will drop the signal indication to a "stop" or a "stop and proceed". If it's a S&P the train will be limited to 15mph, looking out for a broken rail.
I reckon 99% of the derailments (or more) are due to mostly random incidents.
Broken journal, a recent one where a wheel came loose from the axle, or that iron ore train out in California that ran away on Cima grade (that one may be due to a crew miscalculation).
Trains will stop coming off the tracks right after the planes stop falling from the sky and ships stop sinking.