The problem that I see with the requirement for proper application of hand brakes, when called for, is that it is a lot of work to apply them. And since they are redundant to the air brakes, I can see the possibility of rationalizing away the need to apply hand brakes to the full requirement of the circumstance.
A person might decide that, between hand brakes and air brakes together, adequate holding will surely result; rather than seeing it properly in that each system must be capable of doing the holding independently of the other.
I wonder how often that kind of rationalization has been cited as the cause of wrecks or runaways in the history of railroading. It would be an interesting statistic, but I do not know where it can be found.
I do know of one fatal runaway that occurred in Upper Michigan in the early 1950s, and was caused by ignoring the importance of hand brakes while relying only on the air brake. I posted the story here about a year ago. When nearly stalling while climbing L’Anse Hill, they cut the diesel road engine off to run ahead and break the snow and sand the rails for about 1000 feet.
Meanwhile, a steam helper on the hind end kept a push on the train with steam in his cylinders. When the diesel returned, he cut the air in, but apparently had lost most of the car reservoir charges. As soon as the brakes released, he shot backwards. The engineer of the steamer thought the road engineer was shoving back to get to a better place to start, and he worried about flattening the drivers on the steamer, so he stopped resisting. What happed next was a confusing attempt to set the air, but once they had run back a couple hundred feet, they were goners.
The rules called for hand brakes to be set when they cut the road engine off, but the crew did not see the necessity.