Monitoring the conditions of railway tracks is essential for ensuring railway safety. In-service vehicles equipped with sensors and a Global Positioning System device can act as probes to detect and analyze real-time vehicle vibration. Such trains are known as “probe vehicles.” Recently, a compact onboard sensing device has been developed. This paper describes a track condition monitoring system that uses a compact onboard sensing device and diagnosis software. The diagnosis software provides the function of detecting track irregularities using the root mean square of the car body's acceleration. It also allows analysis in the time-frequency domain using wavelet transform. A monitoring experiment on a local railway line showed that the system is effective for practical applications.