System intended to be used for welding an end of a first rail portion with an end of a second rail portion, the first rail portion extending along a first axis and the second rail portion extending along a second axis, the system comprising: - a first frame and a second frame; - means for sliding connection of the first frame with the second frame along a longitudinal axis of extension of said two rail portions according to which the first axis and the second axis are intended to be aligned with one another and with the longitudinal axis; - first clamping means for clamping the first rail portion carried by the first frame and configured to establish at least one free position and at least one clamping position of the first rail portion; - second clamping means for clamping the second rail portion carried by the second frame and configured to establish at least one free position and at least one clamping position of the second rail portion; - means for urging the ends of the rail portions one against the other, - first alignment means for aligning the first axis of the first rail portion along the longitudinal axis carried by the first frame and second alignment means for aligning the second axis of the second rail portion along the longitudinal axis carried by the second frame. characterised in that the urging means are connected to the first frame by means of the first clamping means and to the second frame by means of the second clamping means so that the actuation of the urging means causes a clamping of the first rail portion by the first clamping means and a clamping of the second rail portion by the second clamping means, and comprising a first linear actuator and a second linear actuator each connected to the first clamping means and to the second clamping means, and in that: - the first clamping means comprise a first jaw and a second jaw both rotatably articulated on the first frame and connected to the urging means, the first jaw and the second jaw being coupled in rotation with one another by means of first synchronising means of the rotational speed of said jaws (34a, 34b), - the second clamping means comprise a third jaw and a fourth jaw rotatably articulated on the second frame and connected to the urging means, the third jaw and the fourth jaw being rotatably coupled to one another by means of second synchronising means of the rotational speed of said jaws (36a, 36b), - the first jaw is rotatably articulated on a first end of the first linear actuator, - the second jaw is rotatably articulated on a first end of the second linear actuator, - the third jaw is rotatably articulated on a second end of the first linear actuator, - the fourth jaw is rotatably articulated on a second end of the second linear actuator. - a first frame and a second frame; - means for sliding connection of the first frame with the second frame along a longitudinal axis of extension of said two rail portions according to which the first axis and the second axis are intended to be aligned with one another and with the longitudinal axis; - first clamping means for clamping the first rail portion carried by the first frame and configured to establish at least one free position and at least one clamping position of the first rail portion; - second clamping means for clamping the second rail portion carried by the second frame and configured to establish at least one free position and at least one clamping position of the second rail portion; - means for urging the ends of the rail portions one against the other, - first alignment means for aligning the first axis of the first rail portion along the longitudinal axis carried by the first frame and second alignment means for aligning the second axis of the second rail portion along the longitudinal axis carried by the second frame. - the first clamping means comprise a first jaw and a second jaw both rotatably articulated on the first frame and connected to the urging means, the first jaw and the second jaw being coupled in rotation with one another by means of first synchronising means of the rotational speed of said jaws (34a, 34b), - the second clamping means comprise a third jaw and a fourth jaw rotatably articulated on the second frame and connected to the urging means, the third jaw and the fourth jaw being rotatably coupled to one another by means of second synchronising means of the rotational speed of said jaws (36a, 36b), - the first jaw is rotatably articulated on a first end of the first linear actuator, - the second jaw is rotatably articulated on a first end of the second linear actuator, - the third jaw is rotatably articulated on a second end of the first linear actuator, - the fourth jaw is rotatably articulated on a second end of the second linear actuator.