The whole car/train thing. For me, it depends on the amount of actual cars on a train. For a Eurofighter, I know it's made up of two single articulated cars, but I still call the whole unit a car and not a train. Needless to say with something like a B&M invert, I say a train. I guess it can be, at times, subjective but there are occasions when saying 'car' is stupid. But that's really pedantic, regardless.