Longitudinal member (10) for amphibious undercarriage, which internally defines one or more air chambers (2) for its own buoyancy and externally at least one sliding trail (4), wherein the longitudinal member (10) comprises one or more tracks (6, 6′), which develop around the sliding trail (4) and comprising chains (8, 12) flanked to each other and slidably mounted, each chain (8, 12) comprising concatenated links (14, 16), forged in one piece, fluid-powered motor means, attached to the longitudinal member (10) and configured to promote the sliding of the track (6, 6′) or plurality of tracks through the chains (8, 12). The forged links (14,16) are forged in one piece. The invention further relates to an undercarriage and an amphibious vehicle.