An air distribution system for distributing conditioned air inside a vehicle, notably a railway vehicle, comprising at least: - a first duct having a first air intake for introducing conditioned air inside the first duct, a first conduit and a second conduit which extend, along a longitudinal axis, in a first direction and in a respective opposite second direction with reference to the first air intake, wherein said first air intake comprises a first splitter configured to split the incoming conditioned air into a first flow conveyed inside said first conduit, and a second flow conveyed inside said second conduit; and - a second duct having a second air intake for introducing conditioned air inside the second duct, a third conduit and a fourth conduit which extend, along said longitudinal axis, along said first direction and said respective opposite second direction with reference to the second air intake, and wherein said second air intake comprises a second splitter configured to split the incoming conditioned air into a third flow conveyed inside said third conduit, and a fourth flow conveyed inside said fourth conduit, characterized in that said first duct and said second duct have each a shaped body comprising a plurality of separate modules (M1, M2, M3, M4) assembled together to form the respective first duct and second duct wherein said first duct comprises a first central portion having an inclined side and said second duct comprises a second central portion having a respective inclined side, and wherein, once installed, said first duct and said second duct are positioned side by side with the inclined side of the first central portion adjacent to and in contact with the inclined side of the second central portion, or with the inclined side of the first central portion facing to and spaced apart from the inclined side of the second central portion. - a first duct having a first air intake for introducing conditioned air inside the first duct, a first conduit and a second conduit which extend, along a longitudinal axis, in a first direction and in a respective opposite second direction with reference to the first air intake, wherein said first air intake comprises a first splitter configured to split the incoming conditioned air into a first flow conveyed inside said first conduit, and a second flow conveyed inside said second conduit; and - a second duct having a second air intake for introducing conditioned air inside the second duct, a third conduit and a fourth conduit which extend, along said longitudinal axis, along said first direction and said respective opposite second direction with reference to the second air intake, and wherein said second air intake comprises a second splitter configured to split the incoming conditioned air into a third flow conveyed inside said third conduit, and a fourth flow conveyed inside said fourth conduit, characterized in that said first duct and said second duct have each a shaped body comprising a plurality of separate modules (M1, M2, M3, M4) assembled together to form the respective first duct and second duct wherein said first duct comprises a first central portion having an inclined side and said second duct comprises a second central portion having a respective inclined side, and wherein, once installed, said first duct and said second duct are positioned side by side with the inclined side of the first central portion adjacent to and in contact with the inclined side of the second central portion, or with the inclined side of the first central portion facing to and spaced apart from the inclined side of the second central portion.