Shelf Couplers – never heard of them how about you?  Kadee makes them since 2004, offering #118 & #119  with the differences being their shank length.  Prototype tank cars typically have them to keep the cars coupled together even during a derailment to keep one car from puncturing another.  So what are they?  They are a metal bump on the top and bottom of the coupler that prevents the coupler from riding up or down and become uncoupled due to bad track work or other things that may cause an incidental and unwelcome uncoupling.  These couplers are useful on long freight cars and especially long passenger cars.  I became acquainted with them by accident as I was trying to solve a problem with a new set of Bachmann Passenger Cars I received for Christmas last.  One of the cars had the coupler boxes so poorly mounted to the end of the car that the car would uncouple itself regularly on track that normally caused no issues.  I plan to equip all my passenger cars with the shelf couplers and I have several locomotive pairs that have a tendency to disconnect themselves so they are candidates for the shelf couplers.  As far as I can tell, you don’t have to put Shelf couplers on both sides of the cars being joined, but if you do, they don’t seem to interfere with each other.  One downside, the cars are a bit difficult to uncouple without a skewer or a magnetic uncoupler.

Shelf Coupler (#118 #119)

Typical multipurpose Coupler #148 
(#5 with cat whiskers)

 #119 Metal "SE" Shelf "Whisker" Coupler  #148 Coupler