Ballast, typically composed of gravel-sized natural crushed aggregates with uniform gradation, is the most important structural layer of the ballasted railway track. It supports the rail track structure due to its high shear strength, significant load-bearing capacity, and excellent drainage properties. However, ballast undergoes a gradual and irreversible fouling process by various fines, including coal and subgrade soil, throughout its service life. Coal spillage from freight wagons, during transportation, is the primary contributor to this fouling process. As the fouling progresses, the ballast’s strength and drainage capacity are compromised, leading to potential issues such as differential settlement and reduced track stability. Accurately estimating the degradation mechanisms caused by coal fouling in the ballast layer could significantly reduce maintenance costs for ballasted tracks. In this context, a series of large-scale direct shear tests were conducted to investigate the shear and deformation behaviors of clean and coal-fouled ballast. Experimental results show that coal fines act as a lubricant, causing ballast grains to displace and rotate, thereby reducing shear strength and accelerating deformation. Additionally, the friction angle (φ) of coal-fouled ballast decreases as the void contamination index (VCI) increases from 0 to 5%. However, the breakage of fouled ballast was found to decrease due to the presence of coal fines.