I'm not sure I believe you that anyone would carry a machine gun on a train - the long barrels would make it impossible to even turn around on a train.
That is Charles de Gaulle Airport, but I have seen the same type of soldier at train stations in France. Regardless of what it's called, that is the type of weapon they have.
And I think that when we demand respect towards proper terminology from people in other fields, we should have the courtesy to do the same. It is similar to calling a monitor a PC or a computer case a hard drive. And being afraid of weapons does not matter here, quite many people are afraid of computers also.
Submachine guns are automatic pistols, which is to say they fire lower power pistol ammunition, and, like pistols, usually have shorter barrels than rifles. Typically also have accuracy comparable to pistols rather than rifles.