among professional truckers.. the term "tractor-unit" is fairly common, and this is sometimes abbrev'ed to "tractor."
Professionals, shmofessionals, mate. I'm taking a common or garden as-much-of-an-ignoramus-as-me perspective here.
I'm sure I've heard the front bit of an artic called something else. "Rig" is it?

>a wagon is always pulled by a horse or a team of horses..
or a railway locomotive

Oh yes, as in a "goods wagon". I suppose not so much rail transportation of goods happens these days in the UK.

>>Brits wouldn't have a clue what a panel truck was.
now, I have to disagree with that one, except that we say panel van..better known among commercial drivers and operators than the gen.pub

As I said above regarding professionals...

Actually I can see there is a distinction between an old-fashioned standard (car-sized) van and, say, a Ford Transit van. The latter is more likely to have a door in the side as well as back doors, for one thing.

a truck is more likely to mean a hand cart with four wheels over here
Hmmm, I would tend to think working lorry, as in tow truck, having said which I can't think of any other examples..
Rarely hand cart, but I wonder if this is a North-South thing?

It's a rich area, this, sure enough!