Ballast, typically comprising large sized aggregate particles with uniform gradation, is an essential layer in the railroad track substructure. Functions of ballast include facilitating load distribution and drainage, maintaining track geometry and track stability, and providing track resilience and noise absorption. Throughout its service life, ballast goes through changes in gradation and particle shape properties due to aggregate breakdown/degradation. In United States freight lines, mineral aggregate breakdown/degradation has been reported as the main mechanism causing ballast fouling, which covers up to 76% of all the fouling cases. To investigate the effects of ballast aggregate breakdown and degradation on the mechanical behavior, a series of Los Angeles (LA) abrasion tests were performed to generate fouled materials caused by particle degradation under a controlled laboratory environment. In what follows, large-scale triaxial tests were performed on both clean and heavily fouled ballast specimens using a triaxial test device recently developed at the University of Illinois specifically for testing ballast size aggregate materials. The triaxial testing efforts also focused on (a) the effects of gradation considering those finer materials or fines, i.e. particles less than 9.5 mm (3/8 in.), generated through ballast degradation and (b) the effects of aggregate shape properties, such as angularity and flatness and elongation, for particles larger than 9.5 mm (3/8 in.). Accordingly, triaxial tests were also performed only on those aggregate particles still kept after the LA abrasion tests larger than 9.5 mm (3/8 in.) in size, which would constitute the skeleton of the fouled ballast layer in the field. The experimental study results indicated that ballast degradation did not necessarily result in significant strength loss from the monotonic compression tests on dry specimens. On the contrary, in most cases, the dry degraded ballast with or without fines yielded higher strength properties than those observed in the new ballast specimens. Smaller particles provided a “stabilizing” effect that caused a strengthening of the aggregate matrix and accordingly, fines served as a “stabilizer” to fill the voids and increase density in the ballast aggregate matrix.