railhead (n.)

"horizontal bar passing from one post or support to another," c. 1300, from Old French raille, reille "bolt, bar," from Vulgar Latin *regla, from Latin regula "rule, straight piece of wood," diminutive form related to regere "to straighten, guide" (from PIE root *reg- "move in a straight line").

 In U.S. use, "A piece of timber, cleft, hewed, or sawed, inserted in upright posts for fencing" [Webster, 1830]. Used figuratively for thinness from 1872. By 1830s as "iron or steel bar or beam used on a railroad to support and guide the wheels." To be off the rails "out of the normal or proper condition" in a figurative sense is from 1848, an image from railroads.

Old English heafod "top of the body," also "upper end of a slope," also "chief person, leader, ruler; capital city," from Proto-Germanic *haubid (source also of Old Saxon hobid, Old Norse hofuð, Old Frisian haved, Middle Dutch hovet, Dutch hoofd, Old High German houbit, German Haupt, Gothic haubiþ "head"), from PIE root *kaput- "head."

Modern spelling is early 15c., representing what was then a long vowel (as in heat) and remained after pronunciation shifted. Of rounded tops of plants from late 14c. Meaning "origin of a river" is mid-14c. Meaning "obverse of a coin" (the side with the portrait) is from 1680s; meaning "foam on a mug of beer" is first attested 1540s; meaning "toilet" is from 1748, based on location of crew toilet in the bow (or head) of a ship.

Synechdochic use for "person" (as in head count) is first attested late 13c.; of cattle, etc., in this sense from 1510s. As a height measure of persons, from c. 1300. Meaning "drug addict" (usually in a compound with the preferred drug as the first element) is from 1911.

To be over (one's) head "beyond one's comprehension" is by 1620s. To give head "perform fellatio" is from 1950s. Phrase heads will roll "people will be punished" (1930) translates Adolf Hitler. Head case "eccentric or insane person" is from 1966. Head game "mental manipulation" attested by 1972.