Of course, 'express train' need not imply a machine like a steam engine. In fact even an 'engine' need not necessarily refer to a mechanical device
Quote:
train (n.)
c.1330, "a drawing out, delay," later "trailing part of a skirt" (c.1440), also "retinue, procession" (c.1440), from O.Fr. train (fem. traine), from trainer "to pull, draw," from V.L. *traginare, extended from *tragere "to pull," back formation from tractus, pp. of L. trahere "to pull, draw" (see tract (1)). Train of thought first attested 1651. The railroad sense is recorded from 1824, from notion of a "train" of carriages. British train-spotting "hobby of observing trains and recording locomotive numbers" is recorded from 1958.
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Quote:
engine
c.1300, from O.Fr. engin "skill, cleverness," also "war machine," from L. ingenium "inborn qualities, talent," from in- "in" + gen-, root of gignere "to beget, produce." At first meaning a trick or device, or any machine (especially military); sense of one that converts energy to mechanical power is 18c., especially of steam engines. Engineer "locomotive driver" is first attested 1839, Amer.Eng.
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So a 'train' could be pulled by animals - a 'wagon train' for example, & an 'engine' is simply a 'clever device'.
In other words, one could, in fact, have both 'trains' & 'engines' in M-e without them needing to be even steam powered.