Does anyone remember a company who manufactured a coupler locking device that fit over two mated couplers to prevent accidental uncoupling? I remember seeing their adds in OGR or CTT about 10 or more years ago.
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Your post prompted me to search and I found this. I will be trying this today. We have some Lionel couplers that pop open on a heavy die cast coal train. If you are short on time, start at 2:41.
I just use black rubber bands. 
I also used small rubber bands to keep couplers closed. I had some small rubber bands that my wife had left over from her braces that worked pretty well. But any small rubber bands will do. They're easy to find via an internet search.
This approach requires no changes to the coupler and depending on upon the strength of the rubber band itself allows the coupler to still be uncouple over a UCS track section.
And if 1 rubber band still won't keep the coupler closed...add another one or loop it. Just know this will require a bit of experimentation. Which means...."Oh darn....I have to run my trains!" 
