The degradation of aggregate is the factor that normally determines the service life of the ballast bed. Quite recently the use of crushed rock aggregate has expanded also to the structural layers beneath the ballast bed (subballast). To ensure sufficient frost protection for the track, the thickness of subballast in Finland is normally as much as 1.5-2.0 metres. The ballast degradation occurring in an actual track structure over decades was studied by sampling and extensive series of laboratory tests with a special view to the quality of fines. Many properties of the mineral fines of the analysed ballast samples generally differed relatively little from each other. Degradation of ballast aggregates results mainly from mechanical fragmentation and attrition caused by traffic loads and tamping, and in a few cases possibly from frost weathering. The chemical weathering after crushing of the ballast of Finnish railways is generally insignificant. In the absence of long-term use experiences, the research strived to forecast the degradation of the subballast crushed rock aggregates in cyclic loading apparatus with three separate loading plates. A total of 35 long-term cyclic loading tests equivalent to about 150-300 MGT were carried out varying grain size distribution, strength of the aggregate, amount of fine material and water in the aggregate, loading level and flexibility of the bottom. In the water saturated state the degradation of crushed rock aggregate was dramatically strong. Grading of crushed rock aggregate had a significant impact on the degradation.