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A jim crow is a tool for bending rails, consisting of a U-shaped or V-shaped armature with a hefty screw rod in its axis. Deluxe models are outfitted with rollers for continuous bending. Terry Pratchett uses this term in his novel Raising Steam.
Practice
Railway rails are quite flexible and bend easily to radii of 400m or more. The jim crow is needed to bend rails to tighter radii, especially near the ends.
References
Jim crow (tool) Wikipedia(Text) CC BY-SA