Railway energy supply system for direct current railways, comprising a substation comprising a traction energy source and a return conductor busbar electrically connected to the traction energy source, a return line for the railway return current, which line comprises at least one return line connection conductor electrically connecting the return conductor busbar to a rail return line, and a monitoring apparatus for monitoring the integrity of at least one return line connection conductor, the monitoring apparatus comprising: a current feed device which is designed in such a way that a monitoring alternating current is fed which flows in the at least one return line connection conductor, at least one sensor for detecting a current flowing in the at least one return line connection conductor and an evaluation and control device that evaluates the signal of the at least one sensor and controls the current feed device, characterised in that the evaluation and control device is configured in such a way that the at least one sensor can be used to check whether transients of an operating current flowing in the at least one return line connection conductor exceed a predetermined limit value, or the at least one sensor can be used to check whether a monitoring alternating current flowing in the at least one return line connection conductor exceeds a predetermined limit value, with a loss of a return line connection conductor or a faulty return line connection conductor being inferred if neither transients of the operating current nor the monitoring alternating current exceed a predetermined limit value. a substation comprising a traction energy source and a return conductor busbar electrically connected to the traction energy source, a return line for the railway return current, which line comprises at least one return line connection conductor electrically connecting the return conductor busbar to a rail return line, and a monitoring apparatus for monitoring the integrity of at least one return line connection conductor, the monitoring apparatus comprising: a current feed device which is designed in such a way that a monitoring alternating current is fed which flows in the at least one return line connection conductor, at least one sensor for detecting a current flowing in the at least one return line connection conductor and an evaluation and control device that evaluates the signal of the at least one sensor and controls the current feed device, characterised in that the evaluation and control device is configured in such a way that the at least one sensor can be used to check whether transients of an operating current flowing in the at least one return line connection conductor exceed a predetermined limit value, or the at least one sensor can be used to check whether a monitoring alternating current flowing in the at least one return line connection conductor exceeds a predetermined limit value, with a loss of a return line connection conductor or a faulty return line connection conductor being inferred if neither transients of the operating current nor the monitoring alternating current exceed a predetermined limit value. a current feed device which is designed in such a way that a monitoring alternating current is fed which flows in the at least one return line connection conductor, at least one sensor for detecting a current flowing in the at least one return line connection conductor and an evaluation and control device that evaluates the signal of the at least one sensor and controls the current feed device, characterised in that the evaluation and control device is configured in such a way that the at least one sensor can be used to check whether transients of an operating current flowing in the at least one return line connection conductor exceed a predetermined limit value, or the at least one sensor can be used to check whether a monitoring alternating current flowing in the at least one return line connection conductor exceeds a predetermined limit value, with a loss of a return line connection conductor or a faulty return line connection conductor being inferred if neither transients of the operating current nor the monitoring alternating current exceed a predetermined limit value.