Yup! Speed rail is the stuff the grips used on stage. The modelshop used thicker walled aluminum tube stock so that we could tap screws into it, but we kept the size compatible with the speed rail. The mount rods were lathed to the appropriate size to fit the tube. The speed rail was a little sloppier fit, but worked in a pinch.
Just caught up with this amazing thread!
Back when I was in the industry (80s and early 90s), speed rail was used for damn near everything unless regular scaffolding was called for. Sturdy and highly modular -- for example, the safety railing and other attachments on insert cars are speed rail so they can be quickly reconfigured for the needs of a particular shot. Also camera rigs on picture cars, for the doors and the hood, are usually MacGuyvered from speed rail with special suction-cup attachments (that's not what bears the load though). Given enough speed rail, bungee, and a high hat, those guys could rig a Panaflex to a blade of grass.
It's easier to remember what I never saw it used for -- raising large lights outdoors. That was done with regular scaffolding. After a certain height, I assume a highboy becomes too top-heavy even if you bag the crap out of it (especially if you're raising an arc lamp), so they go to scaffolding and put the stand on top of that.
IIRC, the stuff I saw on set never varied from around a 2", maybe 2 1/2" diameter. I don't remember exactly (though I probably knew at some point, since I was always a gear geek). But it was always exactly the same everywhere I went, obviously because messing around trying to adapt different diameters from different rental houses could be a real time-suck on set.
Wow. Weird what you can suddenly remember after 25 years.
Here's a photo I found of a typical speed rail rig on a picture car:
And here's a couple of insert cars, just swarming with speed rail:
