a synchronous wheel - slip control system for a multi - axle vehicle having a speed sensor for producing signals representative of the velocity of each of the wheel axle units . a differentiator connected to each of the speed sensors for differentiating the velocity signals to obtain rate signals . a plurality of truck and car rate comparators for determining when all the wheel axle units are in synchronism . a plurality of truck and car highest velocity circuits for determining the actual highest velocity signal of the wheel axle units and a car differentiator for differentiating the actual highest velocity signal to obtain the actual highest rate signal . a synchronous slip logic network for causing a data processing circuit to initiate a brake force reduction action of one of the trucks when all the wheel axle units are in synchronism and when the actual highest rate signal exceeds a requested rate .