Five and a half years ago Gargoyle posted (inter alia)
"In Australia, according to A Dictionary of Slang and Unconventional English it meant the baggage car of a Melbourne tram."

As one who grew up in Melbourne I am curious (and somewhat surprised) about the notion that Melbourne trams (what North Americans would call a "streetcar") might have a baggage car. I was familiar with various aspects of the history of Melbourne trams (most of the lines still had the slots for the cable grippers; buggers for bicycle wheels and the tracks were no better) but all the trams I was aware of were for carrying passengers around the city and suburbs, as they still are. "Specialist" trams for carrying baggage or freight were nowhere mentioned or alluded to by anyone I knew or read.

The only "specialist" tram vehicles I ever saw or knew about were cleaners that cleaned out the tracks in the road surface and they only seemed to run in the early morning before the regular services started at sunup. It's possible the "dummy" term applied to such vehicles before my time but I never heard or saw it so used.

When I saw the thread title I confess to a fleeting thought. In Oz, the things parents put into their infant's mouth to quieten them (what North Americans call "pacifiers") are called "dummies". The image of a whole freight train with its payload consisting of dummies was quite entertaining.

Incidentally, the reference to temporary railway tracks through areas being logged reminds me of another etymological variation from Oz, where most of them had the rails made of timber and were known as timber tramways. Some were miles long and thus had a small locomotive pulling the train rather than a "donkey engine" which was a term usually applied only to a stationary (and steam powered) engine.

Cheers, Rowan