train

Behind the everyday word train hides Latin trahere, 'to drag, to pull, to draw' — the same verb that gives English tractor, traction, and abstract. Old French turned trahere into trainer ('to pull, drag along') and the noun train ('a dragging, a trailing line'). Every modern sense grows from that single image: something pulled along behind.

The oldest English meaning is the most literal: the train of a gown — the long trailing part that drags along the floor behind a bride. From 'a thing that trails behind' came 'a connected line of things drawn along,' and when the railway arrived, a train was exactly that: a line of carriages pulled along a track by an engine.

The leap to teaching is the clever part. To train someone is to 'draw them along' a course of practice — leading and pulling a learner forward, the way a gardener trains a vine along a wire, gently drawing it in the right direction. From this verb the whole human family spreads by suffix: a trainer pulls others along (and, in British English, also names the shoe you train in); a trainee is the one being drawn along; trained describes someone the process has finished with; untrained, the one it hasn't touched; retraining is drawing someone along a new path after the first.

trainload is a plain modern compound — as much as a train can carry. trait, sometimes filed alongside, comes from the same trahere source: a feature 'drawn' from one's character, a line of personality pulled out into view.

So whether it trails (a gown), runs on rails (a railway), or shapes a person (to train), the constant is pulling something along.