I'm not convinced that caboose should cease to be counted as a common word. I feel most players, wherever they are, would have heard of the word. The reason it was played by only about one in six of the players in the recent puzzle is probably partly because it's a tricky word to think of.
It has a number of dated and otherwise obscure meanings, but the main one is for a carriage occupied by crew members at the end of a train. As Cal says, the term guard's van is the roughly equivalent term outside North America. But in America or elsewhere, the word is quite likely to be used in a broader sense, often for something going behind other things, such as a sports team that's trailing its competition or a person's backside.
And then there's the classic children's book The Little Red Caboose, written in the 1950s by an American, Marian Potter, but sold around the world, e.g. W.H. Smith in the UK and Big W in Australia.