To enhance the safety and reliability of railway transportation, one of the most important tasks is to check track conditions frequently and accurately. This paper describes a track condition monitoring technique using car-body motions of shinkansen vehicles. A dynamic programming (DP) filter and a Kalman filter were applied in an inverse problem to estimate track geometry in longitudinal level from vertical car-body acceleration and pitching rate measured by an in-service vehicle, proposed solutions were found to be useful for estimating track geometry in longitudinal level with precision even if the vehicle speed is changed.