head of steam

The figurative sense is by metaphor from the literal one: the need of the boiler of a steam locomotive to exceed a minimum amount of pressure before the locomotive could start moving.

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head of steam (plural heads of steam)

1. (idiomatic) A significant amount of energy, vigour or momentum, sufficient to make progress or succeed in a task.
   • 1919, Joseph Conrad, Typhoon:

     "They were keeping a full head of steam, and a profound rumbling, as of an empty furniture van trotting over a bridge, made a sustained bass to all the other noises of the place."

   • 1945 September and October, O. S. Nock, “Wartime Locomotive Working on the G.W.R.—2”, in Railway Magazine, page 255:

     Soon after passing the site of Stretfordbridge Junction Edwards opened out to 25 per cent; boiler pressure was still full up, and Taylor was spreading what was left of the fire so as to arrive in Shrewsbury with only a light head of steam.

   • 1961 March, "Dalmore", “Driving and firing modern French steam steam locomotives”, in Trains Illustrated, page 150:

     [literal sense] Caffiers and Boulogne were passed with a full boiler and a full head of steam, and going up Neufchâtel I couldn't stop her blowing off.

   • 2011 January 25, Paul Fletcher, “Arsenal 3 - 0 Ipswich (agg 3 - 1)”, in BBC‎:

     Arsenal were starting to work up a head of steam and Tractor Boys boss Paul Jewell cut an increasingly frustrated figure on the touchline.

   • 2021 March 10, Greg Morse, “Telling the railway's story on film”, in RAIL, number 926, page 43:

     [literal sense] The film is really about the driver taking control of his charge... the fireman creating a fine head of steam... the signalman keeping the traffic moving safely... [...].

2. Used other than figuratively or idiomatically: see head,‎ steam.