Active wheelset steering has been studied and implemented to improve the curving performance and stability of the wheelsets to overcome the drawbacks of the passive system. A control system for active wheelset steering must be robust to parameter variations and disturbances. A robust sliding mode controller with integral action (SMC + I) for active wheelset steering is therefore proposed and implemented in this paper to control wheelset lateral displacements to achieve perfect rolling conditions during curve negotiation. The robustness of the controller is achieved by deriving the control inputs bounded with known uncertain parameters. The control input is derived as a combination of the equivalent term and a switching term to reduce the amplitude of the switching input. A saturation function is used instead of a sign function in the switching term to provide continuous control. The integral action (I) is added to the sliding surface function to minimize the zero steady-state error. Co-simulation is executed to evaluate the performance and robustness of the designed controller. A conventional railway vehicle with two two-axle bogies with a maximum operating speed of 250 km/h is modelled in SIMPACK®, while the SMC + I controller is implemented in MATLAB/Simulink® for co-simulation. Two hydraulic servo actuators (HSAs), modelled with Simscape hydraulic libraries, are implemented in the longitudinal direction to steer each wheelset. The proposed controller ensures stability, finite-time convergence and zero steady-state errors for all possible running scenarios. This indicates robust performance of the designed SMC + I controller.