One of the main problems in high-speed train transportation systems is the control of the contact force between the catenary and the pantograph collector end. The equivalent stiffness of the catenary is not constant and this leads to contact force oscillations whose frequency and magnitude mainly depend on the train velocity. Such oscillations can originate electric arcs that damage the mechanical structure and reduce the system performance. Here we consider the equivalent stiffness of the catenary to compensate such an uncertainty. Assuming that a sufficiently accurate measure of the actual contact force is available, the use of an output-feedback control scheme based on higher-order sliding modes is proposed. The actual system structure and robustness properties of the sliding-mode control, allow for the almost-complete rejection of the undesired oscillations of the contact force by means of a continuous force acting on the lower frame of an active pantograph. Simulation results are provided.