Steel furnace slag (SFS) and coal wash (CW) are two common by-products from the coal-mining and steel industries in Australia. Rubber crumbs (RC) is a material derived from waste tires contributing to environmental problems in most developed countries. Reusing and recycling these waste materials is not only economically beneficial and environmentally sustainable, but it also helps to address geotechnical problems such as track degradation. In this study, SFS, CW, and RC are blended to explore the feasibility of obtaining an energy-absorbing capping layer with properties similar or superior to conventional subballast. Comprehensive laboratory investigations have been carried out to study the geotechnical properties of SFS + CW + RC mixtures, from which seven parameters (including gradation, permeability, peak friction angle, breakage index, swell pressure, strain energy density, and axial strain under cyclic loading) were used to evaluate the properties of these mixtures used as subballast. It was found that a mixture with SFS∶CW = 7∶3 and 10% RC (63% SFS, 27% CW, and 10% RC) is the best mixture for subballast.