Since this is getting nowhere, I think I can come in now with my WAG. In the early days of the inductrial revolution, they would build a monstrous water wheel and run power shafts through the factory, say a weaving factory, and each loom or tool or whatever would have a pulley (the ones I’ve in museums seem to be made of heavy leather) and when they wanted to bring the machine on line, they would tighten the pulley and use it to power the machine. There is a textile museum in Lowell, Mass., that has a room full of looms powered this way from a large electric motor. I don’t know whether this was converted from a water wheel or if it just didn’t occur to them that it may have been more efficient to have a motor on each loom. Or maybe motors were very expensive and it made more sense to buy just one great big one.
In theory, the first day that Tesla’s Alternating Current plan was put into action was the day that the first power grid went “on line”. As in, alternating current flowing through power lines.
A Wiki page ( which annoyingly I can NOT link to from my iPad searches… 
) reveals that in 1885, a Direct Current line ran some trolly cars.
“The law of Watercourses”, 1854, uses the term “on stream” in reference to a mill.
I guessed this also. In manufacturing industries it is common to speak of doing something “on line” as opposed to “off line”. For example, in web processing industries to do something “on line” means to do it to the web moving past you, on the manufacturing line, as opposed to doing it once the web is wound up into a roll.
The “line” in these references would be a manufacturing line which would literally be equipment largely laid out in a geometrical line, along which the product travels.
Uh, yeah, but that’s simply a location designation such as I mentioned in my most recent post. A watermill is sited “on stream” whether it’s operational or not.
So this isn’t really directly relevant to my quest for the origin of the phrase(s) “to bring on stream/on line”, “to come on stream/on line” as a synonym for making or becoming operational, as a complex system or component.
Thanks all for the additional information!
“The Chemical Age”, 1936
Several uses of “line”
In a production plant the over head pulley system that provided power to machinery.
If the plant had more than one engine that could be put on the line. Or a 2nd boiler could be put on line as engines or more power was needed.
electrical production with more than one generator.
When ships had more than one boiler.
“On the line” has another meaning, that is, to be at risk or in jeopardy: “Would you put your reputation on the line to sign this contract?”
Is this usage related? Is there some common origin for this usage and the modern “on-line” meaning, or are they entirely independent?
(Ok, to be sure, this appears to be a modern translation. Older, more traditional translations don’t use that phrase.)