The reuse and recycling of waste materials has become imperative as the construction industry strives to minimise its carbon footprint and contribute towards sustainable development. Taking into account that aggregates often represent the largest proportion of the materials used in railway foundations, the incorporation of alternative (i.e. recycled) aggregates could significantly reduce the adverse environmental impacts from the construction of such infrastructures. This paper discusses recent advances and challenges associated with the use of recycled C&D materials in the rail track substructure. A comprehensive laboratory study was carried out to assess the relevant physical, mechanical and chemical properties of different types of recycled C&D aggregates and to determine whether their application as alternative fill materials in the railway substructure would be feasible. Test results have shown that recycled aggregates from C&D waste may be suitable for inclusion in embankment and form layer of railway infrastructures. A major challenge related to the use of these recycled aggregates in the subballast layer is their relatively high susceptibility to breakage and wear. None of the recycled C&D materials raised any environmental concerns that could hamper their application in railway foundations.