Champaran HumSafar Express : इन अजनबी सी राहों में, जो तू मेरा हमसफ़र हो जाये, बीत जाए पल भर में ये वक़्त, और हसीन सफ़र हो जाये. - Piyush Singh

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Why is sand used in locomotives?
The locomotive uses a neat trick to increase the traction. In front of each wheel is a nozzle that uses compressed air to spray sand, which is stored in two tanks on the locomotive. The sand dramatically increases the traction of the drive wheels.
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What is a sander on a train?
A sandbox is a container on most locomotives, multiple units and trams that holds sand, which is dropped on the rail in front of the driving wheels in wet and slippery conditions and on steep grades to improve traction.
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Do trains still use sand for traction?
Sanding. On an adhesion railway, most locomotives will have a sand containment vessel. Properly dried sand can be dropped onto the rail to improve traction under slippery conditions. The sand is most often applied using compressed air via tower, crane, silo or train.
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source : internet