Apparatus for monitoring an axle of a wheelset assembly of a railway vehicle, the apparatus comprising a wireless sensor node fitted to a wheelset assembly, the wheelset assembly comprising an axle mounted between opposed wheels, each wheel being fitted to a respective opposite end of the axle, the wireless sensor node comprising a vibration energy harvester for converting mechanical energy from vibration in the wheelset assembly into electrical energy, a sensor for measuring a parameter, and a wireless transmitter for wirelessly transmitting the measured parameter or data associated therewith, and the apparatus further comprising a processor for processing the measured parameter to produce processed data, wherein the sensor is an accelerometer mounted to an end of the axle and the sensor and processor are arranged respectively to measure and process an axle percussion vibration frequency in the form of resonant vibration along the axle.