Abstract: A test signal generator for generating signals simulating the signals obtained from the axles of a railroad car, to permit testing of a railroad car brake controller. During testing of the controller for detection of excessive speed difference between the four axles of a railroad car, the output from a variable frequency oscillator is gated to a first one of the four controller input circuits, while the output from a fixed frequency oscillator is gated to the remaining three input circuits. The output frequency of the variable frequency oscillator is changed to simulate an axle signal indicating to the first input circuit a speed different from the speed indicated to the other three input circuits by the fixed frequency oscillator output. This frequency change continues until the controller indicates an excessive speed difference. During testing for rate of deceleration, the output from a voltage controlled oscillator is gated to a first one of the four controller input circuits.