Converting diecast tractor trailers gets problematic (weight, length) and expensive (the diecast model, the 'Streets "donor" vehicles needed, upgraded motors, etc. can cost upwards of $200 for a large 18-wheeler ) and can be a lot of work - even if it is worth it for what you get.
But the truck shown here, although small and only a ten-wheeler, is what people ask me for most: This relatively simple project costs about $75 total and results in a nice running, good looking, basic tractor trailer. It is the first of three step-by-step projects in the final, Tractor-Trailer, chapter of my second 'Streets book. I wasn't on the forum much yesterday because I spent the whole day from 5 AM to 6 PM, building this model - (don't worry - it would not take that long unless you stop to take a picture and write an explanation at every step.)
It turned out really nicely so I have attached a compacted .pdf of the file describing how to build it below, for anyone who wants it.
For anyone who decides to try this project, it makes a nice truck that runs well on my layout. But I stress that it is nearer the limits of what can be done with a standard 'Streets motor and chassis than one might think: to assure good running it is important keep the trailer you make as light as possible - I took a chance and made a lightweight plastic trailer body but card stock box would keep it even lighter. Lengthening the trailer or making it much heavier will create problems.
EDIT: please note the file is not proofread well and has a few typos - the publisher's line editor will eventually fix them, but for now, I corrected a few here in this updated .pdf just now.