Scientific paper ID 10 : 2003/1

The paper presents an equivalent diagram of a track circuit with one transmitting end and two receiving ends. Two-port-network models have been developed under normal, shunt and broken rail mode of operation. Shunt sensitivity and broken rail sensitivity have been studied in relation to each of the receivers. An algorithm of abstract synthesis used to determine the optimal input impedances of equipment at the ends. The maximum length of the boundless track circuits of the Underground in Sofia has been determined.