head of steam

From the need of the boiler of a steam locomotive to exceed a minimum amount of pressure before the locomotive could start moving. (See head (“buildup of fluid pressure”), also pressure head.)

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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.
   • 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.