Frequency response from load side is urgently desired to hedge against power volatility of renewable energy in modern power system. Electrified railway has become an important load due to the traffic electrification over the past few decades. In this paper, a novel laxity based adaptive frequency control method is proposed to integrate frequency control in train operation, which enables the electric train to provide fast frequency response support to power system while abiding by the train timetable. An online trajectory optimization model considering realistic track gradient and speed limit conditions is developed to rectify the deviation in the guidance curve and to provide power reference for frequency response. The laxity of train operation is newly defined to measure the flexibility that the train can provide to frequency response. Then, an adaptive droop controller is devised based on the laxity of the train operation to provide appropriate frequency response according to current train state without violating the arrival time. Comprehensive numerical examples demonstrate the effectiveness of the proposed frequency response method, and also indicates the capability of electrified railway to support frequency control of modern power system with high penetration of renewable energy.