Hornby #R8219 Coupling Assy Narrow (10pc) Narrow NEM tension lock couplings, which clip into existing sockets on most trains produced more recently. Some trains do not have the socket to accept these types of plugs, as many 4 wheeled wagons don't, so please look for the rectangular socket (Pictured above, in white and arrowed).
They are ideal for replacing couplings on wagons where the couplings have broken, or just replacing because the existing couplings are not adaquite.