In-service vehicles equipped with sensors and GNSS systems can act as probes to detect and analyse real-time vehicle vibration. A compact on-board sensing device has been developed and used for the track condition monitoring system. The diagnosis software provides the function of detecting track faults using the root mean square (RMS) of the car-body acceleration. It also permits analysis in the time-frequency domain using wavelet transform. The actual measured data were obtained from regional railway lines using the developed track condition monitoring system. The degradation level of tracks has been monitored by the system developed and fed back to railway operators to inform on a track maintenance scheme. Field test results in regional railway lines showed that the system is effective for preventive maintenance work.