On the motorway this morning, I passed a motortrike. It was quite large - bigger than a small car - and was pulling a trailer. There were two people on the trike, one behind the other. I don’t know if the rear seat could have held more passengers but the passenger seemed to be centrally positioned. It was sort of in the style of a Harley-Davidson ‘hog’. A bit like this except bigger and with more bodywork. And in a horrid brown colour.
What was it?
Nearly all large trikes on the road are customs. The one in the linked picture is a custom trike based on an air-cooled Volkswagen engine. The one that you saw may have been built around a V-8.
I see Harley trikes around a bit. Look familiar?
Yes, but I think the one I saw was bigger. Maybe an older model?
Again, as Cornflakes tried to tell you, there are no ‘off the showroom’ models for these trikes.
They are almost all custom built. So the ‘what model was it’ question really doesn’t apply.
Actually, Harley-Davidson just started building the Tri Glide Ultra Classic, the first factory trike they’ve ever made (that I’m aware of). However, based on the OP’s description, he probably saw some sort of VW custom.
The ones that freak me out are the ones with two wheels in front, one in the rear. I think they are made by Polaris.
Ooops, I guess it’s actually a Can-Am Spyder by Bombardier.
Boss Hoss ?
http://www.bosshoss.com/
Harley-Davidson made the Servi-Car from 1932 to 1973.
Based on the pictures, it looked more like a Boss Hoss than a Harley. It’s possible that my memory is exaggerating the size of the rear.
Looking at the picture I can see the end can, and that appears to be too small for a twin cylinder machine.
I’d say that it has to be a 4 cylinder engine, highly likely a Japanese engine, my guess is that its from a FJ1200 or something like it. These particualr engines are often lifted and fitted into custom machines
Neither the Boss Hoss nor the Harley trikes look sufficiently powerful to pull a trailer. The trailer wasn’t a caravan, but an ordinary 4’x4’ trailer, and the rear of the trike appeared to be wider than the trailer. So it’s looking like a custom job.
The engine in a Boss Hoss is a 350 Chevy. And there are some models with the 502ci Chevy.
I’m gonna have to say there’s plenty of power there to pull a trailer up the side of a building. 
But odds are that it was some sort of custom job so who knows.
Yeah, I’d guess that half of the 45s around started out as trikes.
