A method of improving or maintaining track systems having a substructure and a ballast bed as a superstructure, said method comprising the steps: removing the ballast of the ballast bed ; removing the material of the substructure up to a desired depth; if necessary, producing a new formation ; installing a new substructure ; producing a new ballast bed , in particular using processed old ballast , characterized by the steps: removing material from a mixing zone in a transition region between the old ballast bed and the old substructure together with the old ballast of the ballast bed ; separating old ballast from the removed material; mixing removed material remaining after the separation of the old ballast with material arising on the processing of old ballast and/or with externally supplied new substructure material; and installing the removed material mixed with processing material and/or external new material for the substructure as a or a part of a new, mechanically improved subgrade layer , and/or formation protective layer and/or frost protective layer and possibly installing a new additional intermediate layer between the new, mechanically improved subgrade layer and the formation protective layer and/or frost protective layer . removing the ballast of the ballast bed ; removing the material of the substructure up to a desired depth; if necessary, producing a new formation ; installing a new substructure ; producing a new ballast bed , in particular using processed old ballast , characterized by the steps: removing material from a mixing zone in a transition region between the old ballast bed and the old substructure together with the old ballast of the ballast bed ; separating old ballast from the removed material; mixing removed material remaining after the separation of the old ballast with material arising on the processing of old ballast and/or with externally supplied new substructure material; and installing the removed material mixed with processing material and/or external new material for the substructure as a or a part of a new, mechanically improved subgrade layer , and/or formation protective layer and/or frost protective layer and possibly installing a new additional intermediate layer between the new, mechanically improved subgrade layer and the formation protective layer and/or frost protective layer . removing material from a mixing zone in a transition region between the old ballast bed and the old substructure together with the old ballast of the ballast bed ; separating old ballast from the removed material; mixing removed material remaining after the separation of the old ballast with material arising on the processing of old ballast and/or with externally supplied new substructure material; and installing the removed material mixed with processing material and/or external new material for the substructure as a or a part of a new, mechanically improved subgrade layer , and/or formation protective layer and/or frost protective layer and possibly installing a new additional intermediate layer between the new, mechanically improved subgrade layer and the formation protective layer and/or frost protective layer . <EOS>