Why are unit train coal hopper cars painted a color on one end? The color itself has no significance. The end with a color denotes that end of the car has a rotary coupler, to enable the car to be dumped at its destination in a big rotary dumper without having to be uncoupled. Outstanding instructions say never to couple two painted ends together, because this is what can happen --- two rotary couplers together can rotate during train movement, and normal train movement can easily cause the knuckles to bypass laterally on curves or switches, regardless in this case of the "tightlock ears." This coal train had several separations during the wee hours, causing emergency stops, recoupling, air test, and proceeding a little farther