A friend of mine collects a lot of monorail sets that were made back in the early 70s. The name escapes me but I can ask if interested. Pretty hard to find today.
The roller coaster sets I posted about in another thread are kind of a variation on a mono. It rides over a single T shaped rail with wheels above and below.
I have a Speedrail Monorail (Remco) set that I got for Christmas in the early '70s along with an add-on set I got off eBay a year or so ago. At the time, the thing ran well, but I haven’t had enough track until last year to run it as an adult, and I haven’t had room to set it up. These sets–both passenger and frieght–can be had on eBay fairly cheap.