A rail tool-holder cart, comprising: A) a support frame having: - a longitudinal beam having two opposite end portions on each of which a roller is mounted for a rolling support of the support frame at two spaced apart points on a first rail, - a cross rod having a connection end rigidly connected to the longitudinal beam in a center region between the two end portions, and a support end opposite the connection end and on which at least one wheel (10, 5) is mounted, for a rolling support of the support frame at at least one point on a second rail transversely spaced apart from the first rail, B) a tool-holder arm comprising: - a turret portion connected to the support frame adjacent to the connection end of the cross rod and rotatable with respect to the support frame about a substantially vertical rotation axis in an unlimited open angular range, - a coupling interface for a removable connection of a tool (17, 17'), - an arm extended transversely to the vertical rotation axis and which connects the coupling interface to the turret portion so that the coupling interface rotates together with the turret portion around the vertical rotation axis and is vertically movable with respect to the support frame between an upper limit position and a lower limit position, C) a spring exerting on the coupling interface a permanent thrust towards the upper limit position so as to compensate for at least a part of the weight of the tool (17, 17'), D) adjustment means for adjusting the spring thrust on the coupling interface. A) a support frame having: - a longitudinal beam having two opposite end portions on each of which a roller is mounted for a rolling support of the support frame at two spaced apart points on a first rail, - a cross rod having a connection end rigidly connected to the longitudinal beam in a center region between the two end portions, and a support end opposite the connection end and on which at least one wheel (10, 5) is mounted, for a rolling support of the support frame at at least one point on a second rail transversely spaced apart from the first rail, - a longitudinal beam having two opposite end portions on each of which a roller is mounted for a rolling support of the support frame at two spaced apart points on a first rail, - a cross rod having a connection end rigidly connected to the longitudinal beam in a center region between the two end portions, and a support end opposite the connection end and on which at least one wheel (10, 5) is mounted, for a rolling support of the support frame at at least one point on a second rail transversely spaced apart from the first rail, B) a tool-holder arm comprising: - a turret portion connected to the support frame adjacent to the connection end of the cross rod and rotatable with respect to the support frame about a substantially vertical rotation axis in an unlimited open angular range, - a coupling interface for a removable connection of a tool (17, 17'), - an arm extended transversely to the vertical rotation axis and which connects the coupling interface to the turret portion so that the coupling interface rotates together with the turret portion around the vertical rotation axis and is vertically movable with respect to the support frame between an upper limit position and a lower limit position, - a turret portion connected to the support frame adjacent to the connection end of the cross rod and rotatable with respect to the support frame about a substantially vertical rotation axis in an unlimited open angular range, - a coupling interface for a removable connection of a tool (17, 17'), - an arm extended transversely to the vertical rotation axis and which connects the coupling interface to the turret portion so that the coupling interface rotates together with the turret portion around the vertical rotation axis and is vertically movable with respect to the support frame between an upper limit position and a lower limit position, C) a spring exerting on the coupling interface a permanent thrust towards the upper limit position so as to compensate for at least a part of the weight of the tool (17, 17'), D) adjustment means for adjusting the spring thrust on the coupling interface.