We have often heard, "my pickup pulls my trailer just fine."  Here is a perspective of why that can be a meaningless statement. I worked for the railroad while I was going to college and I have performed this action, so I speak from experience. A railroad boxcar can easily weigh 100 tons.  You can release all the air from the boxcar's brake system which frees the brakes.  On a level track, a person, me, can push on that boxcar and get it rolling down the track.  This is analogous to pulling a trailer. You then run down in front of that boxcar and try to stop that boxcar from rolling.  I say try, because you will not be able to stop a rolling boxcar with the same capabilities as it took to start it rolling.  This is analogous to stopping a truck-trailer rig. Pulling capability is no indication of stopping capabilities. Now the trailer brakes should be able to stop the trailers weight and the truck brakes should handle the truck weight.  However going down mountains, those brakes, without significant excess braking capacity, can be used to the point of brake failure.  Significant excess braking capacity can come in the form of an exhaust brake or in the makeup of the brakes like you will find in an MDT/HDT truck brakes.