A stationary acoustic sensor emits a sound wave of frequency \(f_0\) through still air toward a train moving directly away from the sensor at constant speed \(v_T\). The wave reflects off the rear of the train and travels back to the sensor. The speed of sound in air is \(v\), where \(v > v_T\). In terms of \(f_0\), \(v\), and \(v_T\), what is the frequency \(f_r\) of the reflected wave detected by the sensor?