SDTC principle

Railway Signalling Track Circuit

Railway Signalling Diagrams

• A Track circuit is an Electrical circuit of which the running Rail of a Railway track forms a part. It is employed for indicating the presence of Trains and thereby for controlling Signalling and Block equipment’s. For the device Track circuit, following truly holds good.

• ‘No single Invention in the history of Development of Railway Transportation has contributed more towards Safety and Dispatch
Control in that field than the Track circuit. Track circuit forms the foundation for the development of practically every one of the
intricate Systems of Railway Signalling in use today wherein the Train itself is continuously active in maintaining its own protection’

• Track circuits are categorised as DC Track circuits, AC Track circuits and Electronic Track circuits (AFTC).



• Open Loop DC TC : The circuit gets completed when the track is occupied through the net resistance of the vehicle axles occupying the track circuit. The series resistance is so adjusted as to give sufficient voltage to the relay when track rails are shunted by axles. In this type of track circuit, if any connection breaks, its occupation goes undetected. Hence, it is used only for limited purposes where its failure does not lead to unsafe conditions.

Open Track Circuit


Closed Loop DC TC: In this track circuit, the series resistance is called a Regulating Resistance. It regulates the relay voltage so that it falls below the drop away value when the track is shunted. The fall is caused by increased voltage drop across the regulating resistance due to rise in circuit current when shunted by the vehicles.

DC Track Circuit


A.C. Track Circuits are provided exclusively in DC traction areas confined to Bombay Divisions of Western and Central Railways. It is possible to work A.C. Track circuits with A.C. traction also, provided the track circuit supply frequency does not have even a harmonic relation with the traction power frequency of 50Hz. 83 1/3 Hz frequency is chosen for this purpose and these track circuits are used in A.C. Traction areas including the places where DC electric traction ends and AC traction starts.

1. Single Rail AC TC
2. Double Rail AC TC

The length of Single Rail Track circuit is permitted up to 500 metres. Maximum permissible length of Double Rail Track circuit is 2,300 metres. The cable lead Resistance at Relay end shall not exceed 12 Ohms irrespective of length of Track circuits. The feed end cable Resistance shall not exceed 12 Ohms for Track circuits of 2,100 and 2,300 metres and it can be progressively more with reduction of Track circuit length.

AC Track Circuit



• The Audio Frequency Track Circuit (AFTC) is the Jointless type of track circuit
• specifically designed to meet the onerous immunity required in AC or DC electrified areas
• against the high levels of interference present mainly due to traction harmonics.
• The equipment is classified as universal since it can be used-in AC, DC or Non-electrified sections and meets all the requirements of the known track circuits.


• The Smartway DTC is a solid-state, fail- safe system which performs, in a safe way, the train detection, rail continuity detection and track to train data transmission functions within a track circuit (TC) using audio frequency signals

Technical Specifications

• TC length: from 20 to 400 m;
• Transversal conductance: from 0 to 0.5 S/km (2 ohm∙km), LTC=400 m;
• from 0 to 1 S/km (1 ohm∙km), LTC=320 m;
• Maximum shunt resistance 0.5 Ohm;
• Overlap length 7 m (joint length);
• Maximum distance between SER and track connections
• 4.5 km if LTC=350 m, Ballast 0.5 S/km (2 ohm∙km);
• 2.0 km if LTC=400 m, Ballast 0.5 S/km (2 ohm∙km);
• 4.5 km if LTC=280 m, Ballast 1 S/km (1 ohm∙km);
• 2.0 km if LTC=320 m, Ballast 1 S/km (1 ohm∙km)
• Cable type 2×1.5 mm2, 40 nF/km, shielded;
• Data transmission speed: 400 bit/s for train detection;500 bit/s for Sacem;
• Rail continuity control throughout track circuit; Yes
• Maximum number of points in TC 2;
• Power supply 220 Vac ±10% at 50 Hz to 60 Hz;
• Temperature range -25°/+70° in the SER; -40°/+80° in the field;
• Frequency carriers (8nos.) 9.5-11.1-12.7-14.3-15.9-17.5-19.1-20.7 kHz;
• Modulation: MSK ±100 Hz at 400 bit/s (Digicode TC)
• : CPFSK ±100 Hz at 500 bit/s ( data);
• Vital Output 12 Vdc on 400 W or 24 Vdc on 1600 W, insulation 1000 Vac;
• DOT 24 Vdc on 1500 W, insulation 1000 Vac;
• Diagnostic RS232 front or RS422/485 rear connector, at 9600 bit/s, packet
• data protocol.

Block Diagram


Frequency and codes arrangement


Frequency and codes arrangement


Electrical Joints


Electrical Joints

Electrical Joints, S-Bond

Electrical Joints, S-Bond



Electrical Joints:terminal Bond

1.Single rail insulation :- Terminal bond allows to delimit the track circuit at a boundary with an Insulated Rail Joint as shown on
Figure The traction current can continue on the next track circuit by flowing through the terminal bond.


2. Double rail insulation :- Terminal bond allows to delimit the track circuit at a boundary with an Insulated Rail Joint on both rails as
shown on Figure.

Double rail insulation





Electrical Joints:Short circuit Bond

Short circuit bond At a boundary with an area without track circuit, a short circuit bond can used as shown on Figure
This configuration does not require any Insulated Rail Joints but the draw back is a dead zone whose length is in relation with the limit shunt value.


SDTC principle

SDTC : specific installation : S Bond in crossing



Railway Signalling Track Circuit, DC TRACK CIRCUIT,Open Loop DC TC, Closed Loop DC TC, AC TRACK CIRCUIT,Single Rail AC TC , Double Rail AC TC, AFTC , SDTC, Technical Specifications, Frequency and codes arrangement , Electrical Joints, S-Bond,Electrical Joints,ELECTRICAL JOINT, TYPE: S BOND , Double rail insulation,ELECTRICAL JOINT, TYPE: DOUBLE ALPHA (TERMINAL ) BOND, INSULATED RAIL JOINT , Electrical Joints:Short circuit Bond,SDTC : specific installation : S Bond in crossing 


Leave a Reply

Your email address will not be published. Required fields are marked *