early 14c., "a drawing out, delay;" late 14c., "trailing part of a skirt, gown, or cloak;" also "retinue, procession," from Old French train "tracks, path, trail (of a robe or gown); act of dragging," from trainer "to pull, drag, draw," from Vulgar Latin *traginare, extended from *tragere "to pull," back-formation from tractus, past participle of Latin trahere "to pull, draw" (see tract (n.1)).,
General sense of "series, progression, succession, continuous course" is from late 15c.; train of thought is attested from 1650s. The railroad sense "locomotive and the cars coupled to it" is recorded from 1820 (publication year, dated 1816), from the notion of a "trailing succession" of wagons or carriages pulled by a mechanical engine.
"to discipline, teach, bring to a desired state by means of instruction," 1540s, probably from earlier sense of "draw out and manipulate in order to bring to a desired form" (late 14c.), specifically of the growth of branches, vines, etc. from mid-15c.; from train (n.). Sense of "point or aim" (a firearm, etc.) is from 1841. Sense of "fit oneself for a performance by a regimen or exercise" is from 1832. The meaning "to travel by railway" is recorded from 1856. Related: Trained; training.