Green’s Dictionary of Slang

jack n.7

[ety. unknown, but note SE jack, used for a variety of machines]

1. a post-chaise, a travelling carriage seating two or four, with the coachman or postilion riding one of the horses.

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[Aus]Vaux Vocab. of the Flash Lang.
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue.

2. a jackboot, a military boot.

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[UK]‘An Officer of the Line’ Military Sketch-book I June c.100: This worthy officer had formed the greaest friendship with the jack-boot of the army [...] he stuck to his jacks and buckskins until the day of his death.

3. (US black) a locomotive, a railroad train.

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[US]N. White ‘Southern Jack’ 🎵 I got a southern jack, all aboard on the southern jack.

In compounds

jack-boy (n.)

a postilion, one who rides one of a carriage’s leading horses rather than riding on the box.

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[Aus]Vaux Vocab. of the Flash Lang.
[UK]Metropolitan Mag. XIV Sept. 333: There's two inside and one on the dickey, besides the two Jack-boys. I took my prad tight in hand, then placing myself in the middle of the road, with my barking-iron pointed at the Jacks.