Track circuit with continued distance monitoring and broken rail protection

1. A track circuit for a railroad track block, said track circuit comprising:

a first occupied track device connected to rails of the railroad track block at a first portion of the block; and

a second occupied track device connected to the rails of the railroad track block at a second portion of the block, said first and second occupied track devices being configured to:

detect a presence of a train on the rails of the block using a DC function, and

once the presence of the train is detected, determine an amount of unoccupied track behind the train using an AC function,

wherein said first occupied track device is calibrated in accordance with an impedance of the second occupied track device, and said second occupied track device is calibrated in accordance with an impedance of the first occupied track device, and

wherein said first and second occupied track devices are configured to determine one or more broken rails in the block, while the train is within the block, based on one or more impedance readings along the rails of the track.

2. The track circuit of claim 1 , wherein the DC function comprises transmitting DC coded signals along the rails of the block and detecting the presence of the train within the block based on the DC coded return signals along the rails.

3. The track circuit of claim 1 , wherein the AC function comprises transmitting low frequency AC signals along the rails of the block and determining the amount of unoccupied track behind the train based on impedance characteristics of AC return signals along the rails.

4. The track circuit of claim 3 , wherein the AC function further comprises determining an amount of unoccupied track ahead of the train based on an impedance characteristic of AC return signals along the rails.

5. The track circuit of claim 1 , wherein each occupied track device comprises:

a transmitter connected to the rails of the block, the transmitter being controllable to transmit DC coded signals along the rails of the block during the DC function and to transmit low frequency AC signals along the rails of the block during the AC function; and

a receiver connected to the rails of the block, the receiver being controllable to receive DC coded signals along the rails of the block during the DC function and to receive low frequency AC signals along the rails of the block during the AC function.

6. The track circuit of claim 5 , wherein each occupied track device further comprises a control unit connected to the respective transmitter and receiver, said control unit being adapted to detect the presence of the train within the block based on the DC coded return signals along the rails and to determine the amount of unoccupied track behind the train based on impedance characteristics of AC return signals along the rails.

7. The track circuit of claim 6 , wherein each control unit is further adapted determine an amount of unoccupied track ahead of the train based on an impedance characteristic of AC return signals along the rails.

8. A method of monitoring a railroad track block, said track method comprising:

providing a first occupied track device at a first end of the block;

providing a second occupied track device at a second end of the block;

calibrating the first occupied track device in accordance with an impedance of the second occupied track device;

calibrating the second occupied track device in accordance with an impedance of the first occupied track device;

performing a DC function to detect a presence of a train on rails of the block; and

once the presence of the train is detected, performing an AC function to determine an amount of unoccupied track behind the train; and

detecting one or more broken rails in the block, while the train is in the block, based on one or more impedance readings along the rails of the track.

9. The method of claim 8 , wherein performing the DC function comprises:

transmitting DC coded signals along the rails of the block; and

detecting the presence of the train within the block based on the DC coded return signals along the rails.

10. The method of claim 8 , wherein performing the AC function comprises:

transmitting low frequency AC signals along the rails of the block; and

determining the amount of unoccupied track behind the train based on impedance characteristics of AC return signals along the rails.

11. The method of claim 10 , wherein performing the AC function further comprises determining an amount of unoccupied track ahead of the train based on an impedance characteristic of AC return signals along the rails.

12. An occupied track device adapted to be connected to rails of a railroad track block, said occupied track device comprising:

a transmitter adapted to be connected to the rails of the block, the transmitter being controllable to transmit DC coded signals along the rails of the block during a DC function and to transmit low frequency AC signals along the rails of the block during an AC function;

a receiver adapted to be connected to the rails of the block, the receiver being controllable to receive DC coded signals along the rails of the block during the DC function and to receive low frequency AC signals along the rails of the block during the AC function; and

a control unit connected to the transmitter and receiver, said control unit being adapted to detect the presence of the train within the block based on the DC coded return signals along the rails and to determine the amount of unoccupied track behind the train based on impedance characteristics of AC return signals along the rails,

wherein the occupied track device provides an impedance,

wherein the occupied track device is calibrated in accordance with an impedance of another occupied track device, and the other occupied track device is calibrated in accordance with the impedance of the occupied track device, and

wherein said first and second occupied track devices are configured to determine one or more broken rails in the block, while a train is within the block, based on one or more impedance readings along the rails of the track.

13. The occupied track device of claim 12 , wherein the AC function further comprises determining an amount of unoccupied track ahead of the train based on an impedance characteristic of AC return signals along the rails.