I think I have found the reason the magnetic couplers uncouple all by themselves when the train is long. If you examine the place where the pin keeps the knuckle closed, you will see that it is slightly tapered. Under vibration and tension on the knuckle, this makes the pin walk along the knuckle until the knuckle opens. I could cause the pin to walk up this slope by putting tension on the knuckle and wiggling it. I took a small cutter and a Dremel tool and ground a notch in the knuckle in the area where the pin engages the knuckle. I made the notch parallel to the pin. After I made the notch, I could not wiggle the knuckle and make the pin move to the uncouple position. I am going to modify a number of cars to test this idea, but it will be a few weeks before I get to it.
Couplers uncoupling in long trains
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Well Don, he does say O-scale and he posted in the O-scale forum, so I'm guessing he's talking about the O-scale couplers. 
I've taken a number apart and done the same thing, I flatten the surface and also smooth it to remove any molding marks from the knuckle. I got a number of electrocouplers to work butter smooth, coupling and uncoupling, I was very pleased with the result.
I've taken a number apart and done the same thing, I flatten the surface and also smooth it to remove any molding marks from the knuckle. I got a number of electrocouplers to work butter smooth, coupling and uncoupling, I was very pleased with the result.
Some guys use rubber bands to keep the couplers closed, but that defeats the purpose of a knuckle coupler. I just had a truck that was not attached to a car and started looking at it and discovered the tapered part where the pin tries to keep the coupler closed.
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I'd like to see pictures of how you mod them without taking the knuckle out.
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