The running gear on-line monitoring system is an on-line real-time monitoring device developed to ensure the operation safety and quality of urban rail transit vehicles and to evaluate the health status of components. Compared with the axle temperature monitoring system, it can detect abnormal conditions of components at an early stage of faults, improve operation safety, promptly inspect and improve the operating environment of components, and extend the service life of components. It addresses issues such as unclear component status of the bogie, lack of health status data, and absence of system-supported data for intelligent operation and maintenance. This system adopts a multi-parameter diagnosis mechanism that combines temperature, vibration, and impact monitoring, along with an expert system, to comprehensively monitor the status of key components of the train running gear, the status of typical rail damages on the track, derailment monitoring, and the running quality of the vehicle.