From 1650s as "moving from place to place;" by 1814 as "having the power of moving by itself. The noun meaning "engine which travels on rails by its own power" is from 1829, short for locomotive engine, which is attested from 1814. A locomotive engine used without rails was a traction engine, which became tractor.
1819, "ten-legged animal, type of crustacean having ten legs" (crabs, lobsters, shrimp), from French décapode (1806), from Modern Latin Decapoda (animalia), from Greek dekapoda, neuter plural of dekapous "ten-footed" (see ten + foot (n.)). From 1885 in reference to a type of locomotive with ten driving-wheels.
early 15c., "fixed standard of measure" (surname Gageman is early 14c.), from Old North French gauge "gauging rod" (see gauge (v.)). Meaning "instrument for measuring" is from 1670s; meaning "distance between rails on a railway" is from 1841.
Railway-gage, the distance between perpendiculars on the insides of the heads of the two rails of a track. Standard gage is 4 feet 8 1/2 inches; anything less than this is narrow gage; anything broader is broad gage. The dimension was fixed for the United States by the wheels of the British locomotive imported from the Stephenson Works in 1829. [Century Dictionary]