In railway tracks, ballast fouling due to fine material mixing has been identified as a challenging issue nowadays. In this research, deformation characteristics of fineballast mixtures were studied from laboratory and a 3-D DEM triaxial compression tests on sand-gravel mixtures. First, void ratio characteristics of sand-gravel mixtures were studied. Then, triaxial compression tests were conducted on specimens of 50% and 80% of relative densities. DEM simulations were done with clumps particles, whose shape was selected using shape indexes of the actual particles evaluated by an image analysis. The results showed that 30% sand specimen showed the smallest void ratios (i.e., the densest packing) in DEM simulations, resulting the highest deviator stress. The results also showed that clump particles gave similar stress-strain curves as those of laboratory specimens.