Crushed stones, known as ballast, are placed alongside rail tracks to provide a stable base for the tracks. The ballast distributes the weight of the train evenly across the track bed, and it also helps to drain water away from the track. Additionally, the crushed stones act as a barrier, preventing the track from shifting or buckling. Ballast is usually made up of crushed rock, such as granite or limestone, and is typically replaced every 10-15 years.