The high-speed train is typically powered by the catenary wire suspended from poles or towers along the rail. An important issue of the high-speed train's power supply is how to set the precise contact force between the pantograph and catenary wires in real time. It is not a trivial task to investigate the pantograph-catenary contact force regulation in engineering practice since the model of a pantograph cylinder is nonlinear and time-varying with dead zone and time delays. In this paper, we propose an adaptive extremum seeking based contact force control strategy for pantograph-catenary systems. The contact force can be regulated in real time without any knowledge of the pantograph cylinder model. Both simulation and experiment results are provided to illustrate the effectiveness of the design.