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I'm aware of the general origin of "a head of steam" being from old steam-powered trains, but how does it break down? I'm guessing that the "head" is the surplus of motive force necessary to break free from a standstill; does some older definition better meet that meaning? The closest seems to be

a body of water kept in reserve at a height; also : the containing bank, dam, or wall

Is there a more direct fit or does it rely on analogy?

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No analogy. It's an engineering term. Head is a measure of pressure. Head is measured in units of distance, (typically feet in the US) and is the height of a column of fluid (usually water) that exerts an equivalent pressure at the bottom of the column.

For example here is a table of head vs psi: enter image description here

So when the engineer wants to get the train moving he needs to build up enough 'head' (steam pressure) to move the train.

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Interesting. Any idea why they picked 'head' for that term? Seems like an odd word choice. – Lynn Mar 16 '12 at 1:24
I don't know for certain but I surmise it comes from measuring pressure at a wellhead. – Jim Mar 16 '12 at 2:36
@Lynn,Jim: I don't know any more than you guys, but anatomically for humans the distance from your "head" to the ground is the same thing as "height". And I note this Wiktionary Talk lumps "head of pressure" in with "crisis coming to a head" as related "meanings". But I'm inclined to think wellhead is a somewhat different sense - more akin to, say, bulkhead. – FumbleFingers Mar 16 '12 at 4:24
@FumbleFingers you may be right. My thinking was that head as in the head of a stream or river is where it originates and comes out of the ground, the wellhead is where the water (or oil) comes out of the ground. The pressure is typically highest at the head and reduces after it leaves the mouth. So I can easily see people asking how much "head" they've got when asking about the pressure at the wellhead. But as I say, it's purely conjecture at this point. – Jim Mar 16 '12 at 5:55
@Jim: I'm not an engineer, but my understanding of physics is the amount of head you have relative to the wellhead depends on how much higher or lower you are than that point. Basically, height is always what counts, whatever it's relative to. – FumbleFingers Mar 16 '12 at 6:09
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