What is a Coded Track Circuit The AAR

• What is a Coded Track Circuit? The AAR defines a coded track circuit as a track circuit in which the steady rail current is interrupted a predetermined number of times per minute so as to form a code consisting of uniform recurring impulses of rail current. • How long have they been in use? The first installation of coded track circuits on steam-operated territory was made between Lewistown and Mt. Union, Pa, on 20 miles of a four-track main line in 1934. The average length of the track circuit was 5201 ft.

• What are some of the advantages over steady state DC track circuits? • Improved shunting sensitivity. • Improved broken-rail detection. • Increased average track-circuit length, double that of non-coded DC track circuits. • Improved protection against the effects of foreign currents or failure of insulated joints. • Elimination of cut sections. • By using different codes of different number of impulses per minute, the current in the rails can be employed to perform functions other than detecting the presence of a train and broken rails. • Cab signaling.

Simplified coded track circuit B TR CF 0. 3 Ohm Code Following Relay Code Transmitting Relay N CT Standard code rates are 75, 120, and 180 cycles per minute

Synchronous Rectification using Master Decode Transformer TR CF Code Following Relay Master Decode Transformer B N TR Slow Pickup/Drop Relay HR Home Relay (HR) remains picked up regardless of the code rate.

Coding stops when track is shunted TR CF Code Following Relay Master Decode Transformer B N TR Slow Pickup/Drop Relay HR drops out

Non-Coded DC Track Circuit In a non-coded DC track circuit, a train shunt must reduce the rail potential below the track relay’s drop-away voltage in order to obtain track relay release. DN-11 Neutral Relay 4 TR Pickup = 0. 280 V Drop Away = 0. 148 V Pickup voltage is almost twice that of the drop away voltage.

Coded DC Track Circuit In a coded track circuit the shunt only needs to reduce the rail potential below the track relay’s pick-up voltage to stop the track relay from following code. Code Following Relay B CF TR Code Transmitting Relay N CT

Decoding of a specific code using Decode Unit 75 CPM Code Rate Decode Unit for 75 CPM TR CF B N TR DR HR Distant Relay (DR) picked up because code rate matches Decode Unit rate.

When code rate does not match Decode Unit 120 CPM Code Rate Decode Unit for 75 CPM TR CF B N TR DR HR Relay drops out because of code rate mismatch.

• What can different codes be used for? By using different codes, block signals can communicate with each other through the track. • Why is this advantageous? It reduces or eliminates the need for a separate pole line saving the railroads lots of $$$$.

Typical Pole Line

Typical Niles Canyon Pole Line Seed Delivery Service

Standard Code Rate Indications • 180 CPM = Proceed • 120 CPM = Approach Medium • 75 CPM = Approach • No Code = Stop

Single Direction Coded Track Circuits 180 CPM

Single Direction Coded Track Circuits 180 CPM No Code

Single Direction Coded Track Circuits 180 CPM No Code 75 CPM

Single Direction Coded Track Circuits 180 CPM No Code 75 CPM 120 CPM

Single Direction Coded Track Circuits No Code 75 CPM 120 CPM 180 CPM

Single Direction Coded Track Circuits 75 CPM 120 CPM 180 CPM

Centralized Traffic Control (CTC) Dispatcher e de Co n Li de Co Control Points Li ne

Centralized Traffic Control (CTC) Dispatcher sets the traffic direction e de Co n Li de Co Control Points Li ne

Reversible Coded Track Circuits for CTC WEST EAST No Coding WCTPR WTR No Coding ECTPR CF CF Both signals monitor their home blocks. ETR

Reversible Coded Track Circuits for CTC WEST EAST DC No Coding WCTPR WTR ECTPR CF Dispatcher sends commands to both control points indicating the traffic direction is eastbound. The west Control Point sends a steady state DC signal which energizes WTR. CF ETR

Reversible Coded Track Circuits for CTC WEST EAST DC DC WCTPR WTR ECTPR CF This causes ECTPR to pickup, through other circuitry, and passes on the DC signal to the next block. This sets the traffic direction. The direction is saved using a polar stick relay. CF ETR

Reversible Coded Track Circuits for CTC WEST EAST No Coding WCTPR WTR No Coding ECTPR CF CF Both signals monitor their home blocks. ETR

Reversible Coded Track Circuits for CTC 120 CPM 75 CPM WEST EAST WCTPR WTR ECTPR CF ETR receives code from east. Code is decoded then sent west. CF ETR

Conclusion • Coded Track Circuit technology has greatly advanced since its initial use in 1934 due to the utilization of modern electronic based hardware. • Complex bidirectional signaling systems can be designed using software and programmed into the control system, simplifying both engineering and maintenance.