English Expression: A head of steam
Before we get to the idiomatic meaning of a head of steam, let me give you the literal meaning. As you probably know, a steam locomotive (SL) boils water to produce steam which then held in an enclosed space. The pressure builds up until there is enough to make the engine work.
Now for the idiomatic meaning. If a person or business has a head of steam, it means that they have built up momentum or a driving force and have started to become successful.
When a person first starts a new endeavour such as a job, business or research project they will start slowly.
As they learn and improve, they gradually build up what is called a driving force or head of steam until finally they are successful and their job, business or research project is running smoothly.