The existing monitoring technologies of railway tracks mainly use the technologies of electrical sensing. These technologies were easy to be affected by electromagnetic fields and external environments and there were potential security risks. Therefore, the on-line monitoring technology of railway track strain based on an identity weak fiber Bragg grating(wFBG) array was used to monitor the occupancy of the track in the real-time. The sensor′s structure of sensing railway strain was designed by finite element simulation and the encapsulation technology of sensors was studied. The strain signal was obtained by detecting wFBG wavelength shift to achieve highly sensitive strain measurement. The identity wFBG arrays were used to verify this sensor and system in the laboratory and field experiments. The results show that the sensor structure can achieve a smaller optical loss, and can ensure that the sensitivity of the sensor reaches 3.4 pm/με, the linearity reaches 0.997 82, and the hysteresis error reaches 0.8%. The online monitoring system of railway track can meet the actual needs of railway operation and management.