>>1605179cost
simple as that
It would cost a load of money to equip every single car with those brakes. Even if you'd skip one, it would at least need the controls, so the ones behind it would work. Then all your locomotives have to be able to handle those, in all variations you'll inevitably get throughout the decades.
And then there is the maintenance cost. Freight cars are at large built to be as simple as possible to keep those low, because complex maintenance-requirements would be a major pain in the logistics-guy's ass.
Those brakes serve mainly as emergency brake systems on passenger trains, especially those that absolutely need to be able to stop quick (HSR, for instance, can shave a lot of stopping distance using stuff like that). For your typical american freight-train, it's pointless. Very rarely are they fast enough for those to make a large difference, and it's even less likely that you'd know to brake in time
>>1605310that's pretty much all they are used for.
