A machine for repairing a ballast bed has an undercutter that defines a direction of movement of the repairing machine, and a distributor for dividing a flow of ballast entering from the undercutter into a rear flow of exiting ballast and a separate front flow of exiting ballast. At least one rear screening machine is positioned behind the undercutter in the direction of movement and is fed by the rear flow of exiting ballast. At least one front screening machine is positioned in front of the undercutter and fed by the front flow of exiting ballast, and the distributor has a variator for varying the ratio between the separate first and the second flows of exiting ballast.