The study deals with the fouling of crushed rock aggregate used as ballast and subballast in railway track as well as the factors influencing degradation and frost susceptibility of degraded aggregate. The degradation of ballast is assessed primarily by analysing extensive amount of samples from the ballast bed. Other examined properties besides grading are strength, water absorption and freeze-thaw resistance of coarse particles and water adsorption, specific surface area, pore size distribution, mineralogy and loss on ignition of fines. Based on observations the degradation of ballast in the Finnish railway network is largely due to mechanical breakage and abrasion of ballast grains from traffic loading and tamping. Chemical weathering of ballast samples proved generally insignificantly small, and where it occurred, it was limited to the surfaces of grains. The frost susceptibility of aggregates is examined extensively by the developed frost heave test arrangement. The practical results of the research include recommendations for the strength, mineralogy and grading of the crushed rock aggregate in the frost protection and intermediate layers (subballast) of track structure.