The invention discloses a responder, and belongs to the field of railway communication signal equipment. The responder comprises a responder onboard equipment communication interface unit and a responder roadside electronic equipment interface unit, wherein the responder roadside electronic equipment interface unit is provided with a responder information loopback interface module; the responder onboard equipment communication interface unit and the responder roadside electronic equipment interface unit are in communication link to send message data to onboard equipment; and the responder roadside electronic equipment interface unit and the responder onboard equipment communication interface unit of the responder are in communication link to connect and communicate with roadside electronic equipment, to receive message data output by the roadside electronic equipment and transmit the message data to the onboard equipment through the onboard equipment communication interface unit, and to output and transfer information reflecting working conditions of the responder and activation to the roadside electronic equipment through low-frequency signals generated by a responder information loopback interface module. The responder can feed back the signals accurately, and the communication distance can reach more than 4 kilometers.