Test device for an electro-pneumatic brake system of trains which has a test-device-side connecting device of an interface, with at least one first contact and one second contact, for connecting to a brake-system-side connecting device of the interface, a switch with a first and a second switch position for connecting the at least first contact and second contact to one another, and a display device which is connected in series with the switch, characterized in that the test device additionally has a first component which allows current to pass in one direction of flow and blocks current in an opposing direction of flow, a second component which allows current to pass in one direction of flow and blocks current in an opposing direction of flow, and a first resistor and a second resistor with different resistance values, wherein the first resistor and the second resistor are connected to the first contact via the first component, the first component is connected to permit a flow of current between the first contact and the second contact in a first direction, the second component and the display device are connected in series in such a way as to permit a flow of current between the first contact and the second contact in a second direction which is opposed to the first direction, and the switch is embodied as a changeover switch and is connected to the first contact and the second contact in such a way that in a first switch position it connects the first contact to the second contact via the first component and the first resistor, and in a second switch position it connects the first contact to the second contact both via the first component and the second resistor and via the second component and the display device. a test-device-side connecting device of an interface, with at least one first contact and one second contact, for connecting to a brake-system-side connecting device of the interface, a switch with a first and a second switch position for connecting the at least first contact and second contact to one another, and a display device which is connected in series with the switch, characterized in that the test device additionally has a first component which allows current to pass in one direction of flow and blocks current in an opposing direction of flow, a second component which allows current to pass in one direction of flow and blocks current in an opposing direction of flow, and a first resistor and a second resistor with different resistance values, wherein the first resistor and the second resistor are connected to the first contact via the first component, the first component is connected to permit a flow of current between the first contact and the second contact in a first direction, the second component and the display device are connected in series in such a way as to permit a flow of current between the first contact and the second contact in a second direction which is opposed to the first direction, and the switch is embodied as a changeover switch and is connected to the first contact and the second contact in such a way that in a first switch position it connects the first contact to the second contact via the first component and the first resistor, and in a second switch position it connects the first contact to the second contact both via the first component and the second resistor and via the second component and the display device.