If you’re doing HO scale, you may have better luck asking in the Model Railroader forum, but there are companies who do suspended roadbeds for just that application. Flip through the current issue of GR, and you’ll find four or five companies advertising there.
BTW, I’ve got an unopened Mantua Super Bowl Express set if you’re interested. I forget which year it goes up to, but I’ve had it for a while, so probably somewhere around Super Bowl 25.
I’ve never seen it done in HO scale because the train would be hard to see when running overhead.
The following companies advertised suspended rail systems for G-scale in the August issue of Garden Railways magazine. You can check to learn whether or not they have anything for HO scale.
I model in both HO and G, and the biggest problem I foresee with trying to use HO for an elevated railroad is the difficulty in keeping track clean. Because they are so much lighter in weight, HO locomotives are much more sensitive to dirty track problems, and I would never consider trying to run HO on an elevated road where the track would be difficult to reach for cleaning.
A non-operating static diaplay along a narrow shelf on one wall may be a better choice, or have a back-and-forth single-track layout along the shelf.