Ballast is the most common material supporting the rail track structure due to its large shear strength, high load bearing capacity and free-drainage. However, the fouling of ballast effects deterioration of track performance, and eventually demands track cleaning. Fouling refers to the progressive intrusion of fines into the ballast layer and subsequently filling the voids. Coal spilling from wagons during transport, sleeper and ballast degradation and soil pumping from soft subgrade are the major factors contributing to ballast fouling. Queensland rail network suffers mainly due to ballast breakdown and intrusion of coal fines. The maintenance costs of ballasted tracks can be significantly reduced if an accurate estimation of the different types of fouling material and associated degradation mechanisms in the ballast layer can be quantified. Moreover, modern track design should be able to capture particle breakage and adjust the ballast type and gradation for improved performance. A series of large-scale triaxial tests were performed with the objective of studying the effects of coal fouling on the shear strength of ballast. The strength and deformation characteristics were investigated for fresh ballast mixed with different percentages of coal fines. In this study, the shear strength properties are linked to ballast fouling indices to better assess the effect of coal fouling on track degradation. The shear strength of ballast was found to reduce significantly with the increase in coal fines. Practical implications of coal-fouled ballast are discussed with reference to possible fouling mechanisms and associated degradation of the track substructure.