Definitions:

Aspect – any valid visual indication of a signal as displayed to the driver

Aspect sequence – The order in which the aspects of successive signals are displayed to the driver

Signal spacing – The distance between the signal or lineside sign displaying cautionary aspect and the signal or lineside sign at which the train is required to stop.

STANDARD THREE ASPECT SEQUENCES

GREEN – YELLOW – RED SIGNAL

STANDARD THREE ASPECT SEQUENCES

* A driver, running under authority of a green aspect in a three-aspect section of line, shall be warned of a stop aspect by the standard three aspect sequence. the minimum signal spacing distances between the yellow and red aspects for various speeds are set out in GK/RT0034.

* Where distances between successive stop signal is more than the maximum permitted by GK/RT/0034, separate distant signals shall be provided.

* Where the permissible speed of trains is greater than 100mph, four aspect signalling shall be considered.

GREEN – DOUBLE YELLOW – SINGLE YELLOW – RED

STANDARD FOUR ASPECT SEQUENCE

MODIFIED THREE ASPECT SEQUENCE

Where it is necessary to position signals such that the required spacing between a YELLOW and a RED aspect in the standard three aspect sequence cannot be provided, sufficient warning of the stop aspect shall be provided by means of one of the following:

1. An isolated standard four aspect sequence (preferred method)
2. Approach control from red of the previous signal to the signal with sub-standard spacing
3. The imposition of a speed restriction.

MODIFIED THREE ASPECT SEQUENCE

1. ISOLATED FOUR ASPECT SEQUENCE (PREFERED METHOD)

2) Approach control from red of the previous signal to signal with sub-standard spacing

MODIFIED FOUR ASPECT SEQUENCE

STANDARD FOUR ASPECT SEQUENCES

A driver, running under authority of a green aspect on a four aspect section of line, shall be warned of a stop aspect by the standard four aspect sequence.

The minimum signal spacing distances between the double yellow and red aspects for various speeds are set out in GK/RT0034.

Green – Double Yellow – Single Yellow – Red

Where it is necessary to position signals such that the required spacing cannot be provided between a Double Yellow and a Red aspect in the standard four aspect sequence, Double Yellow aspects shall be additionally displayed at one or more preceding signals such that minimum signal spacing distance is provided between the outermost Double Yellow aspect and the Red aspect.

• Green – Double Yellow – Double Yellow – Single Yellow (*) – Red

* Train Protection System fitted as a minimum to this signal

• As an alternative to above – restrict the permissible speed

CHARLES WEIGHTMAN’S RECOMMENDATION

Where it is necessary to position signals such that the spacing between the single yellow and the red aspect does not meet the requirement for the 4 aspect sequence (one third of the applicable signal spacing distance), then one of the following modified aspect sequence shall be adopted.

Green – Double Yellow – Single Yellow – Red/controlled to Single Yellow – Red

Green – Double Yellow – Single Yellow – Single Yellow(*) – Red (*) TPWS

ASPECT SEQUENCE TRANSITIONS

Segregation of three and four aspect sequence:

* The number of transitions between three and four aspect signalling shall be kept to a minimum

* It is permissible for an isolated four aspect sequence to be displayed in an otherwise three aspect sequence

AT THE TRANSITION ONE OF THE FOLLOWING ARRANGEMENTS SHALL BE APPLIED

* THE FIRST FOUR ASPECT SIGNAL DISPLAYS A SINGLE YELLOW UPTO THE NEXT SIGNAL AT RED OR A DOUBLE YELLOW UP TO THE NEXT BUT ONE SIGNAL AT RED

* THE FIRST FOUR ASPECT SIGNAL SHOWS ONLY RED/DOUBLE YELLOW/GREEN AND ADDITIONAL DISTANT SIGNAL IS PROVIDED BETWEEN IT AND THE  NEXT STOP SIGNAL AHEAD

* THE FIRST FOUR ASPECT SIGNAL IS APPROACH CONTROLLED FROM RED TO YELLOW WHEN THE NEXT SIGNAL AHEAD IS AT RED

PROHIBITED ASPECT SEQUENCES

Successive signals shall not simultaneously display the following:

a.A red aspect immediately preceded by any colour light aspect other than a single yellow aspect

b.A single yellow aspect followed  by any aspect other than  red, as permitted in modified sequence

Double line block instrument interface with the panel

The entry of the train onto the block section is jointly controlled by the entry and exit points of the block section. The driver is authorized to proceed into block section by the signal controlling the entry into the section. This working could be the ABSOLUTE BLOCK system stem or AUTOMATIC BLOCK system.

Essentials of Absolute block :
“Where trains are worked on absolute block system “

a) Ntraino  shall be allowed to leave a block station unless Line clear has been received from the block station in advance, and

b) On double lines, such line clear shall not be given unless the line is clear not only upto the first stop signal at the block station at which such line clear is given but also for an adequate distance beyond it .

c) On single, such shall not be given unless the line is clear of trains running in the same direction not only upto the first stop signal at the block station at which such line clear is given but also for an adequate distance beyond it, and is clear of trains running in the direction towards the block section to which such line clear is given. The adequate distance referred shall not be less than 180 Mts

d) The whole of the last preceding train has arrived complete; and all necessary signals have been put back to „ON‟ behind the said train.

Block working on double line section

On request from sending station (station-B) station master at station-A will turn the block handle to „Line clear‟ (L-C) position if he is ready to receive the train. LC contact of the block handle energizes LCR. Line clear indication (LCKE appears). The first stop signal (home) is at „ON‟ (RECR↑). The relay GNSR will pick up & switches on the line circuit BPR at station B to pick up. BPR is a polarized relay, thus is gets attracted to line clear side & Bottom Galvo at receiving station A & top Galvo needle indicator at station B turn to the „line clear‟ position.

LCPR is energized through prolarized relay contact on LC side provided the last stop signal displays the „ON‟ aspect. The advance starter can then be cleared for dispatch of train duly proving that the block section including over lap is clear (VPR-UP). When the train crosses advancer starter, TPIR picks up registering train entering the block section. This is accompanied by an audio-visual alarm at the sending station. The buzzer can be stopped by pressing the „ACK‟ button.

Information is then passed on to station A who will turn the block handle to „TOL‟ position. This sends both the galvos to change to TOL position. SR at station B gets energized & remains in that state.

At the receiving station A, when the home signal 1 is cleared, ZR picks-With the sequential proving of ITP & ITPI, BSR & BSPR pick up. Now when block section is clear on complete arrival block clear indication is given. & the block handle can be brought to „line closed‟ position thereby, completing the block operation.

In the event of failure to turn the handle to TOL upon train entering section, auto TOL feature has been  incorporated. As a result TOLR will be picked up when train arrives, ASM gets auto-TOL buzzer/indication. The station master at station A will be forced to turn the handle to TOL. Then through special contact of TOL, BTSR relay picks up. This will in-turn energise the lock and the instrument can be turned to line clear from TOL. This operation is recorded by an electronic counter in addition to audio-visual alarm. Detailed circuit of the block working is shown below.

• Connected Relays:
• LXLR: This relay picks up by proving all concerned ASR/OVSRs of signals in whose route/overlap the L.C. gate falls are free.(i.e., picked up) and UCRs are de-energized (i.e., route is not set).

• LXRR: It proves that the gate is free to be opened for road traffic (i.e., LXLR is up) and gate button LXN and common (group) slot release button GBN are pressed. It proves permission is given from the panel to open the gate. Its repeater at the gate is LXRPR, the front contact of which gives feed to gate key lock to release it.

• KNLR: Proves gate key is in. i.e., the gate is closed against road traffic, locked and the key is kept in the place at gate lodge to transfer control to panel at the station. It is the relay in station, repeating another relay KN_R at gate site. KN_R picks up after key is deposited at site by gate man.

• LXNR: This relay proves control given to the gate has come back to panel and gate can not be opened. After KNLR picks up, panel operator presses LXN + GRN (Group slot restoration button) and LXNR picks up.

• Fig 1 gives the circuit of LXLR.
• Fig 2 gives the circuit of LXRR
• Fig 3 gives the circuit of LXRPR, a site relay.
• Fig 4 gives the circuit of LXNR
• Fig 5 gives the circuit of key extraction and restoration (i.e., pick up circuit of KN-R at gate location and KNLR at the station).If due to any technical snag, gate key can not be taken out to open the gate road traffic will get piled up at the gate. For this in a sealed box at gate site, an emergency key (EM Key)is kept

• This EM key has no lock .It is free. However it has key in contact. So when it is taken out by breaking the sealed box, its key-in contact breaks. This drops KN-R and KNLR. Since KNLR front contact is proved in all concerned signal circuits ie. UCR, HR etc. signal can not be cleared when EM-key is out to open the gate.

Railway Signalling Points control table :-  The route wise control table does not show the points controlled. Each point is controlled by the point track circuits for track locking so that if any train is moving over the points, the track locking will be effective and the points cannot be operated under the wheels. This aspect is illustrated separately in a points controlled table.

This table includes columns showing the routes which lock the points in normal position and in the reverse and also the crank handle controls. This facilitates a cross-checking with that provided in the route wise control table and also used at the time of testing before commissioning. The point control table for the yard under study is placed.

A track circuit is a vehicle detection device in which the running rails form part of an electrical circuit. The boundaries of track circuit are marked by insulation joints on the rail and rails are bonded at rail joints for better conductivity.

Uses of Track Circuits:
* For detecting the presence of vehicles or absence of vehicles within the limits of the track circuits.
* For locking the point when the train is on the point.
* Trolley protection circuit for axle counter to ensure wheels of easily removable trolleys are not counted.

Closed TC: In this type current is always flowing through the relay. When train comes over the track, the supply to the relay is shunted and the relay de-energizes.

The smallest closed track circuit provided is of 26 meter length. The longest workable track circuit depends on the Ballast Resistance (i.e., Resistance across rails offered by the stone chips placed below the rail to support track), This ballast decides the leakage current. In other words ballast resistance appears across or in parallel with relay coil
resistance.

Open Track Circuit:
Open track circuit is one in which the track relay is normally de-energized and picks up only when train comes on the track.

In this track circuit, any disconnection with train on the track will drop the relay and failure on unsafe side will take place, as the relay will show track is clear under occupation. Hence this track circuit can be used for short length only i.e., 26 Mts. Now a days open track circuits are not used.

Fed over track circuit:
It is a sub division of track circuit. This is generally adopted when it is not possible to work a long track due to inability to maintain prescribed parameters like ballast resistance for fail safe working of track circuit. Instead of dividing it in to independent track circuits, the first track circuit is fed by the usual battery and relay arrangement.

The feed to the second track is taken through the front contact of the track relay which controls the first track and so on. The last track relay can serve to indicate occupancy or clearance of the portions of all track circuits.

In the relay interlocking systems, the safety of train movements in the yard has to be ensured in the various electrical signaling circuits that are prepared for the system. The circuits, therefore, have to be prepared carefully and meticulously and checked thoroughly from the point of view of safety. As it will be cumbersome to prepare the
signaling circuits directly from the signal interlocking plan (SIP). The various safety aspects such as interlocking of conflicting routes, requirements of points for each route, the track circuit controls for the points, the route holding requirements such as approach locking and back or route locking and other controls such as crank handle controls, gate controls, block control and overlap release, etc. are first put in a table called “control table.” Or “selection table” and this table is used in the preparation of circuits.

A 2 tier or even 3-tier checking of the control table is done for the correctness and approved by the approving authority before it is translated into circuits.

The control table is also very much useful at the time of testing the installation before commissioning.

The various columns of the control table are shown in the sheet attached.

In the preparation of the control table, the following points should be kept in view:

When a route is set and locked, it should lock all other conflicting route may be.

A. Directly conflicting route: Route which require all its points in the same position as that of the route which is set and locked.

B. Indirectly conflicting route: Route which require at least one of its points in a different setting from the points of the route which is set and locked.

1. The interlocking between indirectly conflicting route is automatically achieved through the points /points as the point set and locked in one position, say normal for the route set will not be available for the conflicting route which require the point in the reverse position, and therefore, the conflicting route can not be set. For directly conflicting routes, as all the points are required in the same position and are already set, the interlocking can not be achieved through the points. The interlocking has to be specifically provided in the circuits and the same has to be indicated in the control table under the column ”Locks signal routes”.

2. The following points has to be kept in mind for at the time of providing the interlocking:

a. A main signal shall lock the shunt signal ahead and vice-versa.
b. A main signal shall lock the shunt signal below it and vice versa.
c. A main signal shall lock the calling on signal below it and vice versa.
d. A calling on signal shall lock the main and shunt signal ahead and vice versa.

e. A calling on signal cleared from the main line shall lock the advanced starter of the same direction and vice versa.
f. Starter signal at either end of the berthing track shall lock each other.
g. Shunt signal at either end of the berthing track shall lock each other.
h. A Shunt signal and a main signal at either end if the berthing track shall lock each other.
i. Conditional locking shall be provided wherever necessary. All the above interlockings have to be shown in column ”lock signal route “of the control table.

3. The requirements of points, normal or reverse as the case may be, for each route and their detection have to be shown in respective columns of the control table. The point control circuits have to be prepared as per the requirements of the points for the route and their detection have to be proved in the route checking and signal control circuits.

4. Before clearing a signal, the route should be clear of any obstruction including the overlap. This is achieved by proving the clearance of track circuits from the foot of the signal upto the next signal, and the overlap track circuits beyond it. All these track circuits have to be shown in the column “controlled by track circuits” and have to be proved in the signal control relay circuits.

5. Other controls such as “crank handle control” “siding point control “, ”gate control “ etc., have also to be shown in the respective columns of the control table and incorporated in the appropriate control circuits.

6. After the signal is cleared ensuring all the above said safety conditions, the route must be held when the train approaching the signal as well as when the train has passed the signal till such time is clears the last point in the route. The former is achieved by providing approach locking on the signal. For the home signal, as there will be no long approach track circuits for this purpose ,dead approach locking is provided.

This is shown in the “Approach locked by track circuit column as “dead approach” or as “signal off” which means that the approach locking will be effective once the signal is taken off and can be released only after the time delay of 2 minutes. For other signals, this can be provided with the help of track circuits. Already available in the rear of the signal or dead approach can be provided.

7. Once a train passes the signal, the route is held by providing back or route locking. The track circuits from the signal up to the last point in the route which are used for this purpose are shown in the column ”Back locked by track circuits “are proved in route release circuit. So long as the train is passing over these track circuits, the route cannot be released.

8. The overlap is released 2 minutes after the train has occupied the berthing track or after the overlap track circuits are cleared by a run-through train. These track circuits are shown in the control table under the column “overlap releases”

9. Once the train passes through the signal, the signal is immediately replaced to ”ON” by occupation of the first track circuit after the signal. This comes under the column ”signal replaced by track circuit occupied”

10. The selections for the calling on signal are the same as those of the signal above it except that the calling on signal will not be controlled by any track circuit other than the first track circuit after the signal as a calling-on signal can be cleared even if any track circuit in its route fails or the berthing track Is occupied .The first track is included in the control only to put back the signal to “ON”. It must be noted that for the calling on signal, no overlap is set and as much the overlap points will not figure in the points column.

The fact that the calling on signal will clear only after 2 minutes of occupation of the calling on track in rear of the signal is indicated in the “Remarks” column. Even though overlap points positions and the overlap track circuits are not proved for clearing the calling-on signal, shunting or any other movement in the overlap portion is prohibited in the interest of safety.

11. For Shunt signal, the selection is the same as those of main signals. No overlap is required for shunt signal movements. Therefore, overlap point positions and detections are not shown against shunt signal routes.

12. There is no route as such for the last stop signal. It is controlled by the block instruments of the section which is shown in the “remarks” column. The short 2 rail length track circuit after the LSS is shown in the controlled by track circuits column.

Individual Button Description
• GN – Signal Button Red
• UN – Exit or Route Button White or Grey
• WN – Individual Point Button Black
• NWWN – Common Normal Point Button Black with red dot

• RWWN – Common Reverse Point Button Black with red dot
• EWN – Emergency Point Operation Button Blue with red dot
• COGGN – Common Calling-on Signal Button Red with white dot
• EUYN – Emergency Route cancellation Button Red with white dot
• RRBUN – Super emergency route cancellation Button Red with White dot
• GBN – Group Slot Release Button Blue

• GRN – Group Slot restoration Button Blue
• EOVN – Emergency Overlap Release Button Red with white dot
• EGGN – Emergency Signal Cancellation Button RED
• G-31 N – Gate Button Brown
• RRBUSN – Emergency Route Release Signal Key to be operated by Signal Staff

Counters provided on the Panel

• COGGN – For recording Calling-on Signal operation
• EOVN – For recording Overlap Emergency Cancellation
• EWN – For recording Emergency operation of points under Track Circuit failure

• EGGN – For recording emergency cancellation of signal
• RRBUNR – For recording route cancellation under emergency with track circuit failure

• EUYN – For recording route cancellation under emergency without track circuit failure
• Emergency/Normal Crank Handle Counter For recording crank handle extraction

Buttons to be pressed for various operations

• To clear a Main or Shunt Signal: GN + UN
• To clear a Calling-on Signal: Calling-on GN + COGGN. Then leave COGGN, press UN
• To put any signal to ON: GN +EGGN
• To operate a point with no track circuit failure: Individual WN +{NWWN or RWWN}, depending on need to take it to N or R.

• To operate a point under track circuit failure: {NWWN or RWWN} + EWN. Then leave EWN and press individual WN.
• To give control to L.C. gate to open: Gate button + GBN
• To take control back from L.C.: Gate Button + GRN
• To release a route after putting the signal to „ON‟ by pressing GN + EGGN if track circuits are functioning normal.
• GN + EUYN Then after 2 minutes ALSR will pick up. OVSR will pick up thro ALSR front contact. Thus Route and Overlap get released.
• To release a route that does not get unlocked after the train has arrived due to track circuit failure..

• Signal staff has to insert their key in the panel and turn. Then GN + RRBUN will be pressed by ASM. After 2 minutes ALSR will pick up. OVSR will pick up thro‟ ALSR front contact. Thus Route and Overlap get released.
• To release overlap manually, if does not get released after the arrival of the train.
• Press GN + EOVN. After 2 minutes overlap will be released.
• To release a crank handle for manual operation of point: Press concerned point CH N + GBN
• To take the control of crank handle: Press CH N + GRN

Audible Alarms

• Any Button stuck alarm.
• Any point or signal failure alarm.
• To stop the alarm, ASM has to press an Acknowledge button. But visual failure
indication will continue, till failure is attended and cleared.

Indications on the Panel

• Point position indication i.e., Normal or Reverse.
• Signal aspect indication i.e., red or yellow or green or double yellow.
• Route set and locked indication.
• Track occupied indication.
• L.C.Gate indication i.e., closed or open.
• Crank handle position: in or out, free or not free.
• SM‟s Key in or out.
• Point locked or free.
• Which group button has got stuck? i.e., signal/point/common button

Block Indications
• Line clear
• Train passing Advanced starter
• Auto-TOL
• Block section clear

Sequence operation of relays for signal clearance (Refer Fig. 7.02).
• SM‟s Key is in and GN + UN buttons are pressed.

Relays that pickup                         Relays that drop
• SMR, GNR, UNR
• LR
• NWR or RWR                                        RWR or NWR
• NWPR or RWPR                                 RWKR or NWKR
• NWKR or RWKR
• UCR
• ALSR,OVSR,WLR

Relays that pick up                                    Relays that drop
• UHR,UECR
• HR                                                                               RECR
• HECR
• Train passes signal                                                Track relay
• RECR,TSR1                                                   TSR,LR,UCR,HR,HECR
• TSR2 etc., TSSLR
• Train clears last point and fully arrives on berth track.

Sequence of relay operation

Relays that pick up                                           Relays that drop
• TRs, ALSR                                                            TSR1,TSR2 etc., TSSLR
• With time delay OVSR
• TSR,WLR

Relays
Relay is an electromagnetic device which is used to convey messages electrically from one circuit to another circuit through a set of contacts (back or front contacts)and works on the principle of electromagnetism.

Relays are classified as :

According to the source of power applied – DC Relays & AC Relays.
According to mounting – Shelf type & plug-in type.

According to the application. – Line relays & track relays.
According to polarity on coils – Neutral relays, polarized relays and Biased relays

According to contact material – Metal to carbon contact relays (non proved)
– Metal to Metal contact relays  (proved)

According to importance of circuit – Vital relay & non vital relays.
According to immunization – AC immunized DC relays,
Non AC immunized D.C .Relays

Shelf type Relays

* These relays are classified as neutral line and track relays. Both can be of AC Immunized or non AC immunized.

* Contact elements of front contact are carbon and metal for both back & movable contacts.

* Contact resistance for front contact is 0.2 ohms and back contacts are 0.5 ohms.

* Continuous contact current capacity is 3 Amps.

Plug- in type relays
Basic Constructional Features:
* Common plug board for all relays.
* Plug & socket kind of interconnection between plug board and relay.
* Retaining clip to hold the relay firmly.

* Connectors positively locked in the plugboard can be withdrawn by a special tool.
* Terminating wires on the connectors is both by crimping and soldering.
* Registration device with specified code combination to prevent interchanging of relays.
* Relay filled with gasket for moisture protection.

* Maximum numbers of contact are 16 independent metal to carbon type.
* Continuous current carrying of contact: 3 amps.
* Switching capacity : 2 Amps.

Metal to metal contact plug-in type relays
These relays fall under the category of proved type relays. Since metal to metal contact can cause fusion or welding when current is interrupted due to circuit making or breaking, proving of back contact is necessary. Otherwise cutting the power to the relay can not be assumed to drop and break the front contacts (picked up contact due to
exercitation of the relay)

Specification:
Coil resistance — 1260 to 1840 ohms
Rated voltage — 60V DC
Minimum number of contacts — 8 Nos.
Contacts combination — 4F/ 4B; 5F/3B; and 6F/2B.
Contact rating. — 5 Amps

Relays supplied can be of neutral relays, interlocked relays or lamp proving relays. Further these relays may be supplied as Mini group consists of two relays on a base plate pre-wired. Minor group with of 15 relays for route, signal functions. Major group with of 30 relays for point functions.

Railways Signalling Relays | Shelf type Relays |Plug- in type relays 

Conditions to be satisfied before a signal is cleared:
* The concerned buttons or knob has been operated by pressing or turning.
* The points required on the route, the overlap and isolation are set and locked correctly and the route is locked.
* The track is clear on the Route and the Overlap.

* No conflicting movement is taking place or initiated already.
* Any level crossing gate interlocked with the signals of the Station is closed against the route traffic and locked.
* The crank handles controlling the points on the route, overlap, and isolation are manually locked in the proper place and not available for taking to the site of the points.

* Any siding connected to the track on which the movement will take place is set to the correct position to facilitate the signal movement and locked.
* If it is the last stop signal, line clear is already obtained from next station.

Power Supply Requirements :

* Battery for operating the relays in the relay room.
* 24 V DC for Q-series relays.
* 60 V DC for K-50 relays of Siemens.
* Battery for extending feed to the outdoor relays.

* 24 V battery for panel indications.
* 110 V battery for operating the points.
* 110 V AC (stabilized) for colour light / LED signals.
* 110 V AC for feeding battery charger to track feed batteries for track circuits.
* Transformers for stepping down 230 V AC to 110 V DC.

* Battery chargers (230 V AC) for various battery banks.
OR
* 11. 110 V Battery with IPS (Integrated Power Supply)

Types Of  Signalling Cables Used

Outdoor Cables : Multi-core copper conductor with PVC insulation, PVC sheath and
steel armouring.
Size of conductor: 1.5 or 1.6 sq. mm.
No. of Cores: 6, 8, 10, 12, 16, 20 and 30.

Indoor Cables : For the internal wiring of relay in locations and cabin.
The cable is with plain annealed copper conductor PVC insulated, unarmoured flame retarding type.
Flexible multi-strand wire size 3 / 0.75 mm.
Flexible insulated wire size 16 / 0.20 mm.
Multi core each core of size 1 mm.
Multi-core each core of size 0.6 mm.
Multi-core is either 40 or 60 cores
Single-core size – 1 mm.

Outdoor Equipment
* Point Machines
* Colour Light or LED Signals.
* Relays, batteries, chargers, and transformers located in location boxes.
* Track circuits.
* Axle Counter ground devices (the evaluator taking inputs from ground devices is located generally in the relay room or in a separate room in the building housing the relays. The ground devices are connected to the evaluator through a quad copper cable with polythene insulation.)

Stages in Installation and Commissioning of Relay Interlocking System at a Station

* Preparation of the permanent way plan by the Civil Engineering Department showing the position of the track, point, station building, a distance of the adjacent station, kilometres of the station etc.
* P-Way plan is examined by the signal department for the suitability of the signalling and then approved.
* Preparation of signalling plan showing the location of signals, the numbering of points and signals, location of the panel room and relay room, interlocked gates within the station section etc.

* Preparation of selection/control table showing the conditions to be fulfilled for each signal movement with the associated interlocking needed, track circuits to be proved, etc.
* Design of the circuits based on the selection / control table.
* Preparation of wiring diagram based on the circuit diagram approved, contact analysis of relays and marking contact numbers, repeater relays etc.

* Installation of relay racks and wiring of the racks as per approved wiring diagram.
* Testing of the actual wiring done.
* Installation of power supply equipment, completion of outdoor works like track circuits, signals, fixing of point machines etc.

* Conducting simulation test.
* Commissioning by connecting indoor equipment to outdoor equipment after final testing.

Comparison with track circuit.
* To detect the presence of vehicle within a prescribed distance is the role of track circuit.
* Dropping of track relay is due to shorting of rails by the axles of a vehicle train.

Disadvantages of track circuits:
* Either wooden or P.S.C. sleepers needed.
* Influence of B.R. and T.S.R. values (with weather).
* Controlled section length is limited.
* Insulation joints are compulsory or special type of track circuit is needed  joint less working.
* More maintenance due to varying track parameters..

Features of Axle counter:
* It works on magnetic flux variation on a ground device for counting the axles and electronic circuits to evaluate in-count and out-count. To detect the presence of wheel.

Advantages of using axle counter:
* It does not require wooden or PSC sleepers.
* It can cover very long sections up to 15 kms.
* It does not get affected either by flooding or poor maintenance of track.
* It does not require insulation joints and so the track can be long welded.

Application of axle counter:
* Monitoring of berthing tracks.
* Monitoring point zone tracks.
* For automatic signaling system.
* For Intermediate block signalling.
* Level crossing warning system.
* Proving block section clearance.
* At steel bridges in lieu of track circuits.

Various types of axle counter:
* Analog axle counter (amplitude modulation),
* Digital axle counter (phase modulation).

Analog axle counter:
* Single entry-exit system (max. 2-D),
* Multi entry-exit system (max. 4-D) (D = detection point)

Each detection point consists of a pair of TX/RX units.

Operating Principle of Axle Counter

The tack equipment consists of 4 sets transducers (transmitter and receiver) fitted to the track at each detection point, The two sets at each end (detection point) fixed on same rail are staggered to discriminate the direction of traffic.

When power is given to Tx coil the magnetic flux is produced and it links with the Rx coil. The Tx and Rx coils are placed face to face. It produces a voltage in the Rx coil. When the wheel of train passes in between Tx and Rx coil, the magnetic flux linking them is cut and the voltage in Rx coil drops. This change in the level Rx coil is sensed as an amplitude or phase change. Depending upon the particular design, the out put of all the Rx coils (2 frome one end of Axle counter section and 2 from the other end of Axle counter section) are taken to the evaluator located in the station, through Quad cable.

Then the‟ IN „count and „OUT‟ count are derived from the filed equipment and evaluated for comparison. If the in count and out count are equal the evaluator shows „SECTION CLEAR„. If there is any discrepancy in the two counts, (it means some wagons / compartments are left in the section) and therefore the evaluator shows „SECTION OCCUPIED„.

At the same detection point , if some axles enter into the section and then return, the system will show „SECTION CLEAR‟ , if the in and out count are equal .

Trolley-suppression Track circuit :
If a non- insulated push- trolley enters the track detections unit, the two axles will be stored as 2 in counts. The evaluator will show section occupied. The push-trolley can be physically lifted off the track and kept aside. Even though the section is clear the system will show section occupied. To avoid this, a short two rail length closed track circuit is provided at each diction point and the transducer equipment of the axle counter is fitted within the track circuit.

Only insulated push-trolleys are to be used, When the insulated push-trolley passes over the track relay will not drop. In which case, the evaluator will not register the counts . On the other hand, if a train axles passes, it shunts the track and the track relay de-energizes before the axles passes over the transducers in witch case the evaluator will register the count.