So... Long story short, the things take an incredible beating and do occasionally fail despite inspection. "Getting a knuckle" or some variation of the phrase is not limited to the knuckle but can mean the coupler body as well.
Crews are supposed to look for evidence of cracks when inspecting cars and locomotives; not the easiest thing to spot as most cars are already coupled together, add in a bit of poor light, weather, or fatigue and some get missed. The primary indicator is fresh orange rust.
As to where and why failure happens, it varies.
The knuckle itself tends to fail around the hole for the knuckle pin. The coupler itself can fail as well, usually around the knuckle pin or where the coupler body meets the drawbar portion of the casting. The draft gear can fail as well releasing the whole coupler from the car (oh, what a bad day that can turn into).
The failures tend to be in the middle of the train as that is where the peak forces occur when the front and rear of the train are trying to do different things (accelerate/decelerate).
As to why... defective (or worn out) equipment or poor train handling (bad marshalling is also a factor).
Despite the fact that they are designed to take a lot of forces the couplers and knuckles wear out, at some point the castings will crack and get progressively worse until they fail.
A rough or inexperienced engineer can break a knuckle by amplifying the normal forces through poor train handling... add in a train that isn't ideally built (marshaled) and that can go wrong fast.
As to the marshalling, even distribution of loads and empties is best but loads towards the front is how any imbalance should be handled, however in the real world ideals are not usually met. Factor in pick ups and set offs outside of class yards and a train can get to be a mess in a hurry, always remember the crew is looking to get things done in the easiest way they can get away with.
As to DPU, it is a cost that has to be figured in to the operations. Is it worth putting DPU in every train or is it worth taking the risk of getting a knuckle?
