A gas turbine is basically a jet engine that uses the thrust created by the expansion of combustion gases to drive turbine wheels. In a gas-turbine-electric locomotive or power car, the turbine powers a generator that produces electricity to power axle-mounted traction motors like those on electric and diesel-electric locomotives.
The Union Pacific was the only user of large gas-turbine-electric freight locomotives. They were more than experimental on the UP, as they were operated in everyday, heavy-duty freight service from 1952 until 1969. The UP started with 10 gas-turbine-electric locomotives in 1952, and increased its fleet to 55 by 1961. Amtrak also operated gas-turbine-electric lightweight passenger trains, but they used integral power cars rather than separate locomotives.
Basic information on the UP's roster of turbine locomotives, all built by General Electric, is included in the book, "Diesel Locomotives: The First 50 Years," from Kalmbach Books. See pages 183-186. (The UP also experimented unsuccessfully with a coal-turbine-electric locomotive.)
There were also direct-drive steam turbine and steam-turbine electric locomotives. They were all experimental and ultimately considered unsuccessful. For their story, see the following article from "Classic Trains":
Turbines: King Coal's battle against the diesel
Classic Trains Magazine, Fall 2004 page 20
Not a story of successes
( "HIRSIMAKI, ERIC", TURBINE, CTR )
So long,
Andy