In a rail vehicle (100) a control unit (140) controls a set of brake/traction units (101, 161; 102, 162; 103, 163; 104, 164) by control signals (B1, A1; B2, A2; B3, A3; B4, A4) to apply a respective brake/traction force to a respective wheel axle (131, 132, 133, 134 to cause retardation/acceleration of the rail vehicle (100). The control unit (140) obtains a first wheel speed signal (.sub.1) indicating a rotational speed of at least one first wheel (121), and obtains a second wheel speed signal (.sub.a) indicating an average rotational speed of at least one second wheel (122, 123, 124). The control unit (140) produces a first control signal (BF; A1) to the first brake/traction unit (101, 161) such that this unit applies a gradually increasing brake/traction force to the first wheel axle (131) until an absolute difference (|.sub.1.sub.a|) between the first and second wheel speed signals (.sub.1; .sub.a) exceeds a threshold value. The control signals are produced such that an average brake/traction force applied to the at least one second wheel axle (132, 133, 134) is gradually decreased when the brake/traction force applied to the first wheel axle (131) is gradually increased. In response to the absolute difference (|.sub.1.sub.a|) exceeding the threshold value, the control unit (140) determines a parameter (.sub.m) reflecting a friction coefficient (.sub.e) between the wheels (121, 122, 123, 124) and a set of rails (181, 182) upon which the rail vehicle (100) travels.

In a rail vehicle (100) a control unit (140) controls a set of brake/traction units (101, 161; 102, 162; 103, 163; 104, 164) by control signals (B1, A1; B2, A2; B3, A3; B4, A4) to apply a respective brake/traction force to a respective wheel axle (131, 132, 133, 134 to cause retardation/acceleration of the rail vehicle (100). The control unit (140) obtains a first wheel speed signal (.sub.1) indicating a rotational speed of at least one first wheel (121), and obtains a second wheel speed signal (.sub.a) indicating an average rotational speed of at least one second wheel (122, 123, 124). The control unit (140) produces a first control signal (BF; A1) to the first brake/traction unit (101, 161) such that this unit applies a gradually increasing brake/traction force to the first wheel axle (131) until an absolute difference (|.sub.1.sub.a|) between the first and second wheel speed signals (.sub.1; .sub.a) exceeds a threshold value. The control signals are produced such that an average brake/traction force applied to the at least one second wheel axle (132, 133, 134) is gradually decreased when the brake/traction force applied to the first wheel axle (131) is gradually increased. In response to the absolute difference (|.sub.1.sub.a|) exceeding the threshold value, the control unit (140) determines a parameter (.sub.m) reflecting a friction coefficient (.sub.e) between the wheels (121, 122, 123, 124) and a set of rails (181, 182) upon which the rail vehicle (100) travels.