Origin and history of train

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)).

The etymological notion in the word is "that which is drawn along behind." In Middle English the noun was used also of a line drawn by chalk (mid-15c.), a rope used in drawing, one's ancestry, also in cookery in reference to bits of food threaded together. Also see train of thought.

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 connected wagons or carriages pulled or intended to be moved by a mechanical engine.

In mechanics (clocks, watches), "a set of wheels through which motion is transmitted consecutively," by 1797.

The Middle English word is attested earlier (early 14c.) in the now-obsolete sense of "a drawing out, delay," perhaps based on the notion of "a dragging out."

Trains have long been an adjunct of full dress for women, frequently coming into fashion, and seldom abandoned for any length of time ; at times they have reached a length of ten feet or more on the floor. A train of moderate length is called a demi-train. [Century Dictionary, 1895] 

"to discipline, teach, bring to a desired state or condition by means of instruction," 1540s, which probably is extended from the earlier sense of "draw out and manipulate in order to bring to a desired form" (Middle English trainen, attested c. 1400 as "delay, tarry" on a journey, etc.); from train (n.). For the notion of "educate" from that of "draw," compare educate. In reference to the growth of branches, vines, etc. from mid-15c.

The intransitive sense of "exercise, drill, discipline" oneself is attested from c. 1600; specifically as "fit oneself for a performance by a regimen of exercise" by 1832. In reference to animals, "render docile and educate to perform certain tasks or tricks."

The sense of "bring to bear, point, aim" (a firearm, camera, etc.) is attested by 1841. The meaning "to travel by railway" is recorded from 1856. Related: Trained; training.