What's "a head of steam"?

[iirochka]

Please, help me to make out this excerpt:
"The place had already been searched, numerous times. (But the evidence hadn't been found). (Inspector) spent the past two days building a head of steam before he finally pulled out a rare piece of theatrics, ranting and railing that the place could never have been searched properly. The only thing that was going to satisfy him: he was going over and doing it himself... Properly this time, mind."
What had the inspector been doing the past two days? Thanks a lot!

[Clanad]

In this context, the original phrase relates to a steam powered engine... usually a railroad locomotive.  Before the locomotive can move it must first build a "head of steam".  A cold or stopped steam engine must first have a fire started and the water turned to steam before opening valves to have it enter cylinders... So it (the steam) has to build up to a certain pressure before it's usable. This Inspector stewed about the situation and "built up a head of steam", i.e., became agitated before releasing the pressure into action...