Sometimes couplers won't stay closed because the bracket holding the coupler head is bent. The easiest solution may be to obtain a replacement coupler assembly which is called: #480-25 or #480-32 conversion coupler assembly. They install with a pair of needle nose pliers. Last I knew, the cost should be around five dollars.
The magnetic trucks on the flatcar are illustrated on this page of the service manual. The most common cause of these couplers not latching closed is a weak armature spring, causing the vertical pin in the armature plate not to be pulled all the way up into the coupler. The pin is what latches the coupler shut. An explanation of how to remove the baseplate and tighten the spring is illustrated here. If this does not work, the spring and rivet may need to be replaced, or the whole baseplate and coupler assembly can be replaced as a unit.
Larry
You may have some options depending on your particular circumstances; for example, if you have another operating coupler truck on another unused car, you can simply swap them.
Or, after some detective work on why the coupler is inoperative, some handiwork with replacement parts can be attempted. Among other causes, like a bent or broken flat return spring, a broken coupler knuckle spring is a common problem with older equipment. This can be replaced by first removing the rivet holding the knuckle, then inserting the new spring. This can be tricky though, so consider bringing it to your local Lionel repair station as well.
Here is a link to the service manual pages for the trucks/wheels/couplers for your flatcar.
Rob
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