This whole idea started as a way to put up a model train shelf in my office. I was going to go with wood, but then thought, "how about something that'll remain in the environment for long after I've rotted away in the ground." Kidding, kind of. I wanted something styled after a trestle bridge or elevated train system. 

The result is one section of bridge/trestle, some connectors for straight sections, some 22.5 degree corner connectors (4 makes a 90 degree corner), and a myriad of wall brackets for varying distances.

Oh, and I made a "bridge connector" for when I ran into a section that I couldn't connect to the wall. I kinda punted on the bridge. I don't know, it was a total after thought, and I just got whimsical with it.

I attached the wall brackets to the wall using just clear flat push pin thumbtacks that I got off scamazon. 

I built this for an N Scale layout, but it could probably work for HO. I don't have any HO track to test with though.