A railcar presence detector including magnetic field sensors spaced along the length of a rail track for detecting magnetic field disturbances caused by ferromagnetic objects, such as railcars, passing along the rail track. Each of the magnetic field sensors generates an output signal that is received by a control unit. The control unit compares the output signal from each of the magnetic field sensors to a detection threshold and controls the position of a contact member dependent upon the comparison between the output signal and the detection threshold. Each of the magnetic field sensors includes a test device that is selectively operable to modify the magnetic field near the magnetic field sensor to test the operation of the magnetic field sensor. During operation of the system including the magnetic field sensor, the control unit can automatically activate the test device to assure that each of the magnetic field sensors are operating properly.