A method and system for detecting defects to railway wagon wheels and to the rail. A method for detecting a wheel flat, or an event that may cause a wheel flat to develop on a railway wagon, including the steps of a) monitoring at least the longitudinal and vertical acceleration of said railway wagon, and b) concluding that a wheel flat has developed, or that there is a risk of developing a wheel flat, if a specific acceleration pattern is monitored, said pattern comprising a longitudinal acceleration above a first threshold followed by a vertical acceleration above a second threshold. A system for detecting a wheel flat of at least one wheel of a railway wagon and the use of an acceleration sensor mounted on the sprung part of a railway wagon for estimating the wheel flat size of a wheel of the railway wagon.