An apparatus for determining the state of occupation of a track circuit on a railway line provided with a plurality of track circuits adjacent to one another, said circuit comprising a pair of rails (R) formed by parallel metal sectional elements between which it is possible to apply a voltage, each track circuit being separated from the adjacent stretch via electrically insulating joints (G). Said apparatus comprises a transmission block (2) capable of transmitting to the track circuit an a.c. voltage signal, associated to which is a predetermined sequence of at least three codewords that are different from one another.