Edit 10/30/13 - I posted the promised video and some photos requested on details of making these vehciles farther down, this morning.
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I had posted some pictures of the quite nice 1:43 'Streets tractor trailers I built several weeks ago. But I've scraped them (for chassis parts) and am standardizing on 1:50 scale buses and big rigs for my 'Streets conversions. Three reasons:
1) 1:50 looks better -- the roughly 15% smaller scale makes my roads seem 15% bigger 
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2) with 1:43 big rigs only the the wheels from the shorty school bus that used to be offered by Lionel fit and looked good (all others are two small to look right). That little puppy is out of production and getting hard to find. The wheels from WBB's current Ford panel van work well on 1:50 trucks - they are still in production so I can get all I want. 3) I was going to have to make all new chassis anyway because I decided to go to 3:1 reduction gearing in the buses and big rigs that run on my country road. (the 1:43 I made before had direct gearing). As before I use a honking big, full size can motor driving the trailer's axle and the gtractor is just pushed. The motorin the model shown below is big: taken from a MTH scale PS1 F3 that died.
This latest is a splendid runner, and looker. It is a Corgi 1:50 1950's Ford tractor and trailer in Greyhound Van Lines livery. Geared 3:1 with that monster motor, it runs smoothly at any speed up from a scale 5 mph up to 75 mph. Traction to climb up hills is nooooo problem: the trailer, which is diecast and has that big motor, weighs 26 ounces - more than a pound and a half. The tractor weighs 7 oz. I normally run it at a scale 42 mph, even in the country, probably just a tad slow for a US Highway in the early 50s in the foothills of the Rockies.
The Corgi model came with side mirrors and an exhaust to attach but they look cheesy - way too big and bulky, so I will make my own from scratch in a day or two.
This Greyhound van lines tractor-trailer has the identical drivetrain (same part # motor, same gearing) as the Greyhound bus shown and a Trailways bus not shown both (also Corgi). All three run at almost identical speeds at any voltage: my country road is 96 feet around so I space them about 32 feet apart around the loop and they orbit at a scale 42 mph for about 20 minutes before one catches another and I have to intervene.
Why was this the most difficult 'Streets conversion I've done? About half of it is the scale: it's noticeably harder to cram everything inside something that is only 63% of the volume (43/50 cubed = 63% as much room inside). And the rest was just the nature of thisparticular Corgi model, which didn't really want to take the wheel flanges or shorten to exactly fit the wheelbase I wanted or . . . I spent hours carefully grinding here and there with a dremel, knowing onw false step and I would ruin the model's body . . . and the trailer had to be worked on alot to take a powered chassis. But it was worth it. A heavy, nicely finished 'Streets truck with pretty close to a factory "feel" when I hold it that runs as well as I can imagine a 'Streets tractor trailer ever could.
Edit: that's a 1:43 New Ray 1953 Mack on the right, facing the 1:50 Greyhound van lines truck, to show the difference.
