This paper describes the development of a lateral damper designed to improve the running safety of railway vehicles during seismic activity. The developed damper was tested experimentally using a full-scale vehicle model consisting of one bogie and a half carbody on our own large shaking test facility. Results confirmed that the running safety of the vehicle equipped with the prototype damper was better protected against earthquakes than the vehicle equipped with a conventional damper. The performance of the prototype damper in providing better protection to Shinkansen vehicles under earthquake conditions was numerically verified. Finally, line tests carried out confirmed that the developed lateral damper behaved in the same way as a conventional damper under ordinary running conditions.