I lived near rails for a while and they didn't inspect them more than once or twice a month. Also I met a kid some years ago who said his dad was with some agency that investigates derailings, he said they happens a lot and its really easy to derail a train.
Perhaps, but they don't have to break to derail a triain. I probably shouldn't say this but even just a rock next to the rail will do the trick.
It definitely looks deliberately severed, as it goes all the way through the whole rail. If it was a joint it would be centered on the tie. So yeah in this case its probably sabatoge. I was only responding to how often the rails were inspected based on my observation living near amtrak/commuter line for a few years. We had barely any freight trains which are much heavier. Freight trains would rattle the house 300' away.