Sooooo. I posted a "how-it-work" pictorial.
Here it is for your enjoyment
First, the train arrive at the yard. The magnet is under the road bed about 2 inches in front of the loco.
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Next, the train stops over the designated area. You'll notice the couplers are still closed.
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The locomotive pulls ahead slowly, the knuckles are pulled apart by the magnetic field, and the car is uncoupled.
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You can really see the knuckles wide open on this shot.
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The locomotive pulls away from the magnet, and its coupler is back in its "normal" or “centered” position due to the spring in the coupler box. The boxcar knuckle will remain to one side until moved away from the magnet.
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I will do the same steps for uncoupling a boxcar halfway thru the train. After stopping over the magnet, and pulling ahead slowly, the cars are uncoupled
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By slowly backing up, the left boxcar can be used to push the uncouple one away. The boxcar couplers are not aligned due to the magnetic field and the boxcar can be "shunted" into its proper area.
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I find it easier that way. It’s a simple idea that works well.