Most locomotives are equipped with a sandbox that contains (surprise 😉) sand. The sand can be dropped on the rail in front of the driving wheels to prevent the wheels from spinning and to improve traction. This might be necessary due to weather conditions like rain and snow, or on steep grades.
https://en.wikipedia.org/wiki/Sandbox_(locomotive)
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Photo credit: RHaworth/CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Sandbox_270.JPG