I run long trains and I agree the biggest problem becomes couplers. An example is, if your locos hit a dirty spot and jerk then the knuckle snaps off the coupler or the knuckle pops open, be it kadee, mchenry, whatever. I have also broke coupler pockets off and broke the shank on plastic couplers. If it happens in a curve then the center of the train can also be pulled into the center of the curve derailing it. Same with accelerating or slowing down with the train in a curve. And forget about pulling a long train up a curve (like a helix). S curves also play havoc with long trains as do under 30" radius curves ( I use 36 and 38 radius in the big train area). Also dips in the track cause uncoupling big time on long trains. Just a slight dip causes the couplers to work off center and then the pull and casting draft in the couple knuckles causes one to slide up and the other down and they detach (worse with plastic couplers and couplers that are longer than need be). For best results use kadee metals aligned perfectly, use the shortest coupler possible, and paint the coupler knuckle inside with acrylic paint for some surface roughness. Or use horn hooks which are superior in long train pulling reliability. And the longest I run are around 50! FRED