1. In a non-driving railroad vehicle, the combination of wheel and axle means supporting a railroad vehicle having air operated friction braking means, a structurally independent hydrodynamic brAking unit drivingly connected to the wheel and axle means, control means operable by compressed air for selectively switching said braking unit and said friction braking means between braking and non-braking operation, and means including an anti-skid valve in the air line leading to said friction breaking means responsive to the skidding of the wheels upon a surface for releasing both the friction braking means and hydrodynamic braking unit upon being subjected to the action of compressed air when skidding occurs, said control means includes an operating valve in the air line in front of the hydrodynamic braking unit, and means including a control valve in the air line in front of said operating valve responsive to either the vehicle speed decreasing below a predetermined value or the friction braking means being lightly engaged for closing the air line to disconnect the hydrodynamic braking unit whereby said braking unit is inoperative.