A selectively extendable length trolley for supporting and conveying various sized and configured containers for pieces or components in an industrial plant. The trolley includes an adjustable length trolley structure, front and rear adjustable frames and locating supports for the containers that include side supports adjustable in position in a transverse direction, orthogonal to the longitudinal direction (L). A steering control unit (2) of the trolley (1) is articulated to the front frame (32) and operates in such a way that the front wheel unit and the rear wheel unit are made to oscillate in opposite directions to keep the wheels aligned with the curved path to be followed. In a preferred embodiment, the front and rear adjustable frames are T-shaped providing an open or free space at the two sides of the central structure of the trolley.