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I'm not entirely sure about the situation in the USA but in present-day Europe, this certainly isn't true.
  1. The pneumatic braking system of most wagons are equipped with so-called load-proportional valves which adjust the pressure and thus the braking effort relative to the car's total mass. Most of the rest is equipped with hand adjusters for the same purpose.
  2. The purpose of load-proportional braking isn't merely to avoid wheel slip, but also to avoid train ruptures due to uneven braking.
  3. AFAIK the reason freight trains take so long to stop is that the braking system is adjusted for a slow development of full braking force, to avoid train ruptures and large longitudinal forces resulting from one end of the train already being fully braked while another end is still rolling freely. That's because in long trains, due to the relatively low speed of the propagation of the pressure drop in the main brake pipe, it takes a substantial time for all brakes to activate. In recent times however, heavy-haul railways (who run the longest trains) started to equip their rolling stock with electronically controlled pneumatic (ECP) brakes, so faster braking is possible.

*Lunatic*, n.
One whose delusions are out of fashion.
by DoDo on Sat Aug 24th, 2013 at 02:15:55 AM EST
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