A system for steering at least one unpowered wheeled tag axle of a vehicle which in the latter's ordinary direction of movement follows after the vehicle's front axle, comprising • a power cylinder connected mechanically to the tag axle in order, via the position of its piston, to control the alignment of the tag axle's wheels relative to the longitudinal direction of the vehicle, • a pump (13, 53) adapted to conveying a medium under pressure from a medium reservoir (14, 54) optionally to either of two opposite sides (15, 16) of said piston of the power cylinder in order to cause the piston to move and thereby alter said alignment of the tag axle's wheels, and back from the other of these sides in an ordinary feedback line (19, 21, 76, 75), • a power device (12, 52) adapted to running the pump, • a steering unit adapted to controlling the device for control of said alignment of the tag axle's wheels on the basis of information about the alignment of the front axle's wheels relative to said longitudinal direction of the vehicle, and the speed of the vehicle, and to causing the tag axle's wheels to assume a fixed neutral position aligned with said longitudinal direction when a threshold speed of the vehicle is exceeded, and • an arrangement adapted to responding to malfunctions within the system by switching the system to a fail-safe state with inactivated device (12, 52), characterised in that said arrangement comprises an alternative feedback line (28, 75, 76), connected to said both sides (15, 16) of the power cylinder's piston in order to convey medium to the reservoir, and an ancillary unit situated in this feedback line and adapted to keeping this line closed when the system is operating, and that the ancillary unit comprises a connection in series of two valve means (32, 33, 57, 59 and 58, 60) in the form of a first bistable valve means (33, 57, 58) provided with first facilities (34, 61) adapted, when the system is operating, to causing a valve of the valve means to assume either of two stable positions, depending on the vehicle's speed, viz. an open position when the vehicle is below said threshold speed and a closed position when the vehicle is above said threshold speed, and with a second valve means (32, 59, 60) provided with second facilities (38, 40, 69, 70) adapted to switching a valve (39, 71, 72) of the valve means from closed to open position when a said malfunction occurs within the system, so that the ancillary unit is adapted, if the vehicle is below said threshold speed when a said malfunction occurs, to leading medium in the alternative feedback line (28, 75, 76) for caster steering of the tag axle in a first fail-safe state but, if the vehicle speed is above said threshold speed when a said malfunction occurs, to keeping the tag axle locked in said neutral position in a second fail-safe state. • a power cylinder connected mechanically to the tag axle in order, via the position of its piston, to control the alignment of the tag axle's wheels relative to the longitudinal direction of the vehicle, • a pump (13, 53) adapted to conveying a medium under pressure from a medium reservoir (14, 54) optionally to either of two opposite sides (15, 16) of said piston of the power cylinder in order to cause the piston to move and thereby alter said alignment of the tag axle's wheels, and back from the other of these sides in an ordinary feedback line (19, 21, 76, 75), • a power device (12, 52) adapted to running the pump, • a steering unit adapted to controlling the device for control of said alignment of the tag axle's wheels on the basis of information about the alignment of the front axle's wheels relative to said longitudinal direction of the vehicle, and the speed of the vehicle, and to causing the tag axle's wheels to assume a fixed neutral position aligned with said longitudinal direction when a threshold speed of the vehicle is exceeded, and • an arrangement adapted to responding to malfunctions within the system by switching the system to a fail-safe state with inactivated device (12, 52),