This paper summarizes the development of a portable track-condition-monitoring system for easy installation on in-service vehicles. In this system, rail irregularities are estimated from the vertical and lateral acceleration of the car body. The roll angle of the car body, calculated using a rate gyroscope, is used to distinguish line irregularities from level irregularities. Rail corrugation is detected from cabin noise with spectral peak calculation. A GPS system and a map-matching algorithm are used to pinpoint the location of faults on tracks. Field test using in-service vehicle was carried out to evaluate the developed system. The results show that the condition of rail irregularity and rail corrugation can be estimated effectively.