The Bristol Railway Archive

You possibly don't understand that the term is usually meant in a dismissive way. There aren't any classes, they are all kettles, is the underlying meaning.

It would be interesting to know whether diesel captions ignoring classes and referring to all of them as boxes would be appreciated. Unlikely ...

The fact you don't understand rather proves my point really.

Names like Spam Can, Mickey Mouse, Streak, Growler, Whistler, Hoover etc. refer to classes, not an entire form of motive power.
I first noticed 'kettle' around the late 80's, in modern traction mags, from younger fans too young to remember steam.
I think it was derogatory then and, still is from ...