although OED Online(!) lumps all these usages together under the common headword online (on the route of a railway line or airline, computing, engineering; on line, on-line, online), the earliest usage they have is the following:

1926 Econ. Geogr. 2 15 Approximately two-thirds of the coal handled by the system originated at on-line mines, and one-third was received from connections.

edit: this citation is evidently from the Journal of Economic Geography, a peer-reviewed academic journal published quarterly by Clark University since 1925.

here are some later adverbial cites:

1968 Daily Colonist (Victoria, Brit. Columbia) 28 Nov. 13/6 The Skookumchuck mill recently came on line, but none of the B.C.-produced pulp destined for Japan is in the present cardboard packages.

1975 Nature 9 Oct. 435/3 Domestic uranium reserves will be totally committed to those nuclear reactors which are brought on line in the next 20 years.

1989 Accountancy May 151/2 When it went on-line many considered it the most advanced automated assembly unit in Europe.

2000 T. Clancy Bear & Dragon xil. 580 They'd proven to the tanks of the North Vietnamese just how fearsome a foe a missile-armed chopper could be, and that had been before night-vision systems had come fully on line.

BTW, off-line (or offline) evidently predates on-line.

1919 Jrnl. Polit. Econ. 27 331 Another saving consisted in abolishing the off-line offices and setting the traffic department at other tasks.

Last edited by tsuwm; 01/20/11 10:53 PM.