So why did they have to jump off?
......... That was my first thought as well. Three pages into this thread and I still haven't heard an explanation for it.
There's a grade involved, right? If they haven't handled the air brakes correctly (or some other problem that depletes the air) they can lose any ability to control the speed on a grade. We refer to it as "pissing away your air". Once you let that happen, in many cases, jumping is your only option. Usually, it starts and gets critical before you get much over 25mph.
What about dynamic braking?
Rail air brake systems were the basis of what the trucking industry uses today. When you run LOW on service system air, the triple valve on each car senses low air pressure in the 'train line' (supply) and uses the isolated emergency tank to make the brakes apply, independent from the operator...you can't move that car or train until air is restored. It is pretty fool proof...I'm having trouble believing 55 cars failed.
Not how truck brakes work today.
The emergency circuit has air in tanks, that air actually keeps the brakes from
automatically applying.
In a low service air pressure situation, the valve pops and dumps air from the
emergency circuit. That allows hellishly strong springs to actuate the brakes.
What you describe is 50 years obsolete.
That system was air dependent. The default was no brakes.
It's why parked trucks used to roll away, why chocking truck wheels is still a thing.
Combined with engine designs then, It's why an unmanned truck could start and drive itself away. And why no smart driver ever parked a truck in gear.
The current defaults to locked brakes.