trainyoudaoicibaDictYouDict[train 词源字典]
train: [14] A train is etymologically something that is ‘pulled’ along. The word was borrowed from Old French train, a derivative of the verb trahiner ‘drag’. And this in turn went back to Vulgar Latin *tragināre, a derivative of Latin *tragere, a variant of trahere ‘pull’. It was first used in English for ‘delay’, from the notion of being ‘pulled’ back, and ‘part of a garment that trails behind’ dates from the 15th century.

When steam locomotives pulling carriages were introduced in the 1820s, the combined vehicle was called a train of carriages; the simple term train is first recorded in 1835. The use of the verb train for ‘instruct, school’, which dates from the 16th century, evolved from an earlier ‘direct the course of growth of a plant’, which in turn went back to the original notion of ‘pulling’.
=> tractor[train etymology, train origin, 英语词源]
train (n.)youdaoicibaDictYouDict
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 rome 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 first attested 1650s. The railroad sense "locomotive and the cars coupled to it" is recorded from 1820 (publication year, dated 1816), from notion of a "train" of wagons or carriages pulled by a mechanical engine.
train (v.)youdaoicibaDictYouDict
"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.