Trying to stop 21,000,000# plus consist...
(Beware I did not RTFA, if it about commuter train, they are far worse off... fewer contact patches to aid in stopping)
with 12 or 24 (even 36) quarter-sized contact patches (initially until the air brake system generates enough volume and pressure to start stopping the cars too) is like trying to plow New York City during a major snow-storm with a toy beach sand shovel.
@68MPH a consist of 100 cars, each car being quite near ~100 Tons of total weight and the locomotive being 400 tons to 500 tons (sometimes more than one locomotive is at the head end), takes a few feet to stop it, or even slow down.
((100x100)+(450x2))x2,000#=21,800,000#
Let me see, @68MPH with 21.8M#... how many Newtons (Contact Patch downforce on the rail is the limiting factor, but the Brakes also need to produce that amount of braking force) would it need to be able to slow to 0 in ~500ft. Figuring that the cars would start to aid in slowing @ the rate of 2 cars per second. Figuring the amount of time the car gets pushed into the the forward one with the slack, the brakes applying then the slack being taken out the other way)
Ha, the train's last car won't stop for 35 seconds after the locomotive does. But the 2 locomotives cannot generate enough Air Volume or pressure to stop in 500ft which then pushes the locomotive to nearly 1900ft.
So, unless all the brakes on the cars were fitted with electric or instant action (yeah right) air bakes... 500ft cannot be achieve.
Many people forget that trains have a very long reaction time. In fact, you could compare them to Large Oil Super Tankers. You start stopping about 3-10 minutes before you need to be stopped... YMMV.
