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?