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.

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