Vibration analysis of axle box bearings is addressed for monitoring the condition of wagon wheel sets, which is crucial for the safety of high-speed railway traffic. We investigated the application of piezoresistive microelectromechanical silicon sensors designed for low-frequency resonant operation. Sensor fabrication using a low-cost planar technology and characterization are described. We show the feasibility of our method by vibration measurements with wheel sets of high-speed trains under the harsh conditions of railway operation leading, e.g., to intense background interference. Incipient fatigue damage could be detected on the cups of axle box bearings which were operated at constant and variable rotational speed.