A machine for uploading and positioning rails on crossties in railway tracks includes an inverted U-shaped gantry (1-2), with side branches finished at the lower part in respective caterpillar tracks (4) of movement on the ballast of the beam on both sides of the platform of crossties. The gantry (1) supports a pair of clamps (8) with jaws (16), for fixing the rails by the core thereof, and preventing pitching, which clamps (8) close by means of respective hydraulic cylinders (10) movable in height actuated by another hydraulic cylinder (15) through a deformable parallelogram (12), while at the same time they are longitudinally movable by the actual movement of the machine. The machine thus moves forward towards the rail carrier train located at the end of the track under construction, grasps a pair of rails with its clamps, and pulls them without contact with the ground until they are arranged on their assembly position, the rails serving for the forward movement of the rail carrier train.