The system of working of trains is related to maintaining the safe distance between the trains.

Absolute Block System:

Under Absolute Block system of working trains, the entire length of the track between last stop signal of the departing station and the first stop signal along with the block overlap, at the station receiving the train, is maintained clear before starting a train from the station leaving for a ‘B’ class station. While the departure of the train is defined by the train entering into the block section having passed the last stop signal of the station and arrival of the train on the station ahead shall be defined based upon other classifications of the station scheduled to receive the train. For example on an ‘A’ class station line shall be kept clear up to the starter signal of the line on which the train is scheduled to be received. The overlap shall be kept clear up to 180 meters and 400 meters beyond first stop signal under multi-aspect signaling territory and two aspects signaling territory respectively.

Automatic Block System:

The section between two stations is divided into one or more Automatic Block sections each controlled by running train and the aspect of automatic signal ahead while retaining signals of the station on either end of the block section, as it is. An ‘A’ marker painted in black on a circular plate with letter ‘A’ on it, painted white is provided on the post of each such Automatic Signal Post to enable the Loco Pilot to identify the Automatic Block Signals and follow instructions applicable to such signals.

Manually controlled signals within the station section are made semiautomatic to enable a free flow of the trains uninterrupted till required, by an additional signal with letter ‘A’ illuminated by white light. There shall be no light when automatic control is withdrawn, as and when required. During withdrawal of automatic control, such signal stands to work as manually operated signal there being no ‘A’ marker circular disk on such signal post.

Intermediate Block Signaling system:

Longer block sections on double line section are divided into two, by providing an Intermediate Block Stop Signal, proceeded by Distant and Inner Distant Signals as the case may be, enabling pushing of two trains in the undivided block section thereby increasing section capacity. The first section on the departure end of the station is essentially track circuited contiguously between the last stop signal of the station and the Intermediate Block Stop Signal (IBSS) along with overlap section thereon. The other section beyond IBSS and the reception signal of the station ahead, may or may not be track circuit. A telephone connected with the Station Master of the despatching station is provided at the IBSS.

Keeping in view essentials of interlocking clause  of SEM which reads as “It shall not be possible to take “Off” a running signal, unless all points including isolation are correctly set, all facing points are locked and all interlocked level crossings closed and locked against public road for the line on which the train will travel, including the overlap.”

When the train is approaching and the overlap required to be kept clear beyond the first reception signal, before granting line clear, is termed as Block Overlap. When the train is running within station section, the Signal Overlap shall be provided beyond the signal up to which the train leads when the signal in the rear is taken Off.

Block overlap:

For the trains entering into Station Section from the Block Section, in two aspect signaling area, the distance beyond the first stop signal, which shall be kept clear, shall be not less than 400 meters while the same shall be 180 meters if multiple aspect signaling is provided.

Signal overlap:

The track, to be set and kept clear, beyond the next signal, up to which the train is scheduled to lead from the signal being taken Off, is termed as Signal Overlap. This distance is kept clear to ensure that in case the train overshooting the signal, it should not find an occupied track or open points, causing wrong movement, collision or derailment. This overlap shall be 180 meters in two aspect signaling area and 120 meters in case of multi-aspect signaling territory, beyond the trailing point on single line section and beyond the signal up to which the preceding signal leads.

  1. The overlap so provided shall be set towards running lines, Short Siding or the Sand Hump
  2. Short Siding of 180 or 120 meters long in case of two aspect or multiple aspect territories, respectively
  3. Sand Hump As an alternative to short siding a Sand Hump of approved design with at least one rail of 13 meters between point and the sand hump

Long Siding provided for shunting or stabling of hot axle or other damaged wagons or coaches, with a trap point placed at a distance of 180 meters in case of two aspect signaling area or 120 meters in case of multi-aspect signaling area, subject to that the portion of the siding between the signals, beyond which the overlap is required, and the trap point, shall be kept always clear.

In the case when no provision out of above three is available for the overlap to be set, it shall be towards the running line. If two or more routes are there for the departure of the train, the points shall be set for overlap towards the route on which the train is proposed to be despatched, to avoid wastage of time at the time of departure.Gradients in and around the station section

For designing the signaling system for a given yard, the gradient is one of the important factors as the catch or slip siding shall be required to be provided for steeper gradients on either side of the station yard. Adequate protection shall have to be provided in the station section when the gradient is steeper within station section.

Under Schedule of Minimum Dimensions, a gradient steeper than 1 in 400 within station limits is not permitted unless special safety devices are used and/or special rules are enforced. This dimension under recommended Schedule of Dimensions is 1 in 1200. No station yard shall be constructed nor should any siding join a passenger line on a gradient steeper than 1 in 260, except where it is unavoidable and then only with the prior sanction of the Railway Board through Commissioner of Railway Safety, when a slip siding or other arrangement is made sufficient, to prevent accidents.

There shall be no change in grades within 30 meters of any points or crossings except in hump or gravity yards.Beyond 45 meters of the outermost points at the station, with grades steeper than 1 in 400 meters, trains should not be drawn up to the last stop signal and held up on the steep gradient, in order to clear the reception line for giving permission to the following train.

No shunting beyond outermost points on the steep gradient side should be allowed unless a locomotive is attached at the lower end of the lead from the point of view of gradients On hilly area where railway track is having steep gradients, the failure of brake of the train or detachment of a vehicle from the running train can lead to rolling of the train or detached vehicle to the adjoining station placed on lower level and hit a train /vehicle standing there.

To safeguard against such a situation, at a station where there is a gradient of 1 in 80 falling towards the station or 1 in 100 falling away from the station within 45 meters beyond the outermost points at either end, a catch siding in case of 1 in 80 falling towards the station and a slip siding in case of 1 in 100 away from the station should be provided.

Monitoring track vacancy, of a given section of the track:

Monitoring of the track can be done by providing the track circuit which may be any of, DC track circuit, AC track circuit, Audio Frequency track circuit or uneven waveform track circuit superimposed on rails of the track; Axle counter as another electronic device which is not superimposed on the rails of the concerned track but attached to the rail to count-in or count-out the axles of the passing train. Other special feature track circuits such as coded track circuit are also used for the purpose of transferring certain information to the loco pilot running the train with the contact between Loco wheel and the track.

The track circuit is the device to monitor the presence of a vehicle or train on a defined portion of the rail track. As per GR 1.02 (57), the track circuit is defined as “An electric circuit provided to detect the presence of a vehicle on a portion of a track, the trails of the track forming part of circuit”.Types of track circuits to be used in the station yard and in the block section shall be decided based upon monitoring requirements of the track section, type of sleepers provided on the track in station yard and block section.

DC track circuit:

The DC track circuit is most commonly used track circuit owing to it being time-tested, dependability and simplicity of maintenance. DC track circuit is also chosen as the best option in the area provided with AC traction. In station yards where Standard–I or Standard-II of interlocking is proposed to be provided, where if, as a special case semaphore signaling is chosen, only signal replacement portion of track circuits and run-through lines shall be required to be track circuited. On stations provided with Panel Interlocking or RRI, all running lines of the station are required to be track circuited. DC track circuits are also a preferable choice in such a situation.

AC track circuits:

In an area where DC traction is provided, DC track circuits shall not work, as such AC track circuits are provided. Such track circuits are also chosen where stray currents are observed on the portion of a track, proposed to be track circuited.

AC track circuits with uneven pulse4:

The track circuit, as designed by M/s Jeumont Schneider, is known as Jeumont Track Circuit. It has a transmitter with an uneven pulse, and the receiver is tuned to detect and decode the same uneven pulse. This is used where Alternating stray currents, strong enough to interfere with DC track circuit, are observed, on the portion of the track in the station yard which is proposed to be track circuited.

Audio Frequency Track Circuit (AFTC):

As the maximum length of DC track circuit which can be provided on PRC sleepers is 350 meters under normal circumstances, necessitates cutting of both the rails of the track at every 350 meters to provide Rail Insulation Joint (RIJ). In station yards where the 13 feet rails are usually available, provision of RIJs does not pose the major problem. Now that the glued joints are used, replacement of the glued joint poses a problem of traffic block.

As such in block section provided with Automatic Block Signaling, the AFTC is used for the main feature of it being Joint Less. The Nominal frequency numbered ‘A’ to ‘H’ respectively 1699 Hz, 2296 Hz, 1996 Hz, 2593 Hz, 1549 Hz, 2246 Hz and 2445 Hz is used in the system. The nominal frequency works within frequency band varying between +/- 17 Hz. It shall be ensured that no two frequencies are used on adjoining AFTCs. The Track circuit is designed to End Fed ranging up to 200 meters and Central Fed ranging up to 900 meters. The system is designed for 24 V DC working. However, the design may vary from manufacturer to manufacturer. The AFTC is compatible with 25 KV AC traction as such can be used without any limitation.

Axle counters:

A track circuit is electrically superimposed electrical/electronic circuit on rails using them as conductors for which sleepers on which rails are fixed must be such that both the rails are kept insulated from each other. This circuit changes its state when shunted by the wheels of the Loco or any other vehicle. This change in the status of the track circuit gives an indication of the presence of the train. It must be understood here that the circuit is so designed that it does not get short-circuited by the train’s wheels to prevent damage to the power supply for the track circuit. Axle counter is a device with a set of transducers clamped at either end of the section of track which is required to be monitored for the presence of the train, count the axles entering the area of track section and transmit each count to the Central Evaluator which in turn gives an indication that the track section is occupied till the train leaves the track section in question, from the other end or the same end from which the train entered. The out counts are compared by the evaluator comparator and if the numbers of in-count and out-count both match, the evaluator give indication that the track section is clear.

The advantages of using Axle counters are that:

  1. The System is independent of type of sleepers used
  2. No rail insulation joints are required
  3. No outside connections with the rails are required
  4. No limit of length of the track section to be monitored

The disadvantages being:

  • Motor trolley, if put on the track lifting manually without passing the transducers on either end, shall neither clear the track nor be detected.
  • Motor trolley or other wheels dissimilar in shape and size to the wheels of regular loco or other vehicles are sometimes not detected causing failure of the system.
  • Sensors of track monitoring coaches interfere with the system resulting in failure,
  • The axle counter system once failed has to be reset.
  • The resetting can be done with the track section occupied, for which precautionary measures are required to be taken before and after resetting the system.
  • The track circuit provided for monitoring the track, in general, performs certain additional functions and purposes.
Purpose of track vacancy monitoring:

There may be the number of track circuits in any station yard or Automatic Block Signaling Section, depending upon requirement, which is different sections of track with the specific requirement such as to monitor the train:

  1. Approaching the first reception signal
  2. Replacement of the signal back to `On’ having passed the signal and occupied the track circuit.
  3. Track locking the point having occupied the track covering point zone for the purpose.
  4. Placement of the train at the birthing portion of the station yard, temporarily.

Moving ahead of the birthing portion of the yard, where the train occupied the departure track for replacement of the signal at the back of the train and if the track circuit is ahead of Advanced Starter signal, to prove that the train has entered into the block section and initiate process of sequential operation of Block Instrument.

  1. Checking for the Overlap clearance.
  2. Flank protection
  3. Approach locking, back locking, route release & sectional route release.
Device to monitor the signaling system-Data Logger:

The data logger is an electronic device to monitor digital and analog functions happening with the relay based electrical interlocking and operating system. The digital data, generated with each energization and de-energization of the relays earmarked to be monitored, is stamped with each happening, time and recorded into the data logger memory. This information can be retrieved at the relay room where the data logger is provided as well as at the Signal Control Room (Test Room) in the divisional headquarters with which all way station data loggers are connected in parallel.

The data so retrieved is used for predictive maintenance and also in the investigation of accidents to establish time versus activity such as crossing of the signal at red by Loco Pilots etcetera.The analog portion to be monitored is with respect to the level of potential of all sorts of the power supply, with reference to time.

The data logger shall be provided in each cabin/relay room out of which one, the Master Data Logger, shall be connected to the communication channel. In case of multiple cabin arrangement at one station, one data logger shall work as Master and rest of the data-loggers placed in different cabins, termed as Remote Terminal Units (RTU) are connected to the Master Datalogger. The Master Data logger shall transfer the information to the signal control room at Divisional Headquarters Office, through Optical Fibre Cable (OFC) or any of the available communication media/channel.

Data Logger shall be provided at all interlocked stations and should preferably be networked. Monitoring console of networked Data Loggers should be manned either by keeping in Test Room/S&T Control room in divisional headquarters office, by providing signaling staff round the clock so that maintenance engineers may verify and validate the fault logic related to data loggers and also check the calibration of analog parameters.

Anti Collision Devices:

Monitoring devices are i) which monitor only and convey the message to the loco pilot and ii) which not only monitor and convey a message to the loco pilot but also take action in case the loco pilot does not respond in time. Such devices are:

  • Automatic Warning System (AWS)
  • Train Protection & Warning System (TP & WS)
  • Raksha Kavach
  • Train Actuated Warning Device (TAWD) for level crossing gates.
Automatic Warning System (AWS):

The only means of interaction between the Loco Pilot in the Loco and the Station Master in his room at the station is the signal. In a case for reasons what so ever, if the Loco Pilot misses the signal at On, a disaster is imminent. To combat such a situation to arise, Automatic Warning System (AWS) is provided at an adequate distance ahead of the first approach signal which interacts with the sensor provided at the bottom of the loco to interact with a track device tuned to convey the status of the signal being approached, placed in between the rails of the track. The Loco plot getting a warning signal shall take suitable measures to control the train.

Train Protection & Warning System (TP & WS), The system comprising of

  1. trackside subsystem
  2. computer-based system evaluator with an indication panel and a Taco Meter, installed in loco.
  3. The passive track device, the electromagnet AWS is replaced by a sealed microprocessor chip mounted on the sleepers.
  4. This tracking device is an interface between interlocking system at the station and the loco based computer.

A warning is given to the Loco Pilot to enable him to react within 5 seconds of the warning, if the Loco Pilot does not react in time, and the train exceeds 10 kilometers beyond the permissible speed, the service brakes shall be applied till the train is brought down to the permitted speed and if the train is not likely to stop at the given point, to stop the train. The system is for complete control of the train by monitoring

  1. the speed of the train
  2. signal indications in the Loco Cab,
  3. automatic application of brakes when required and
  4. application of temporary speed restrictions en-route.
Raksha Kavach:

Designed under the name of Raksha Kavach is for continuous protection of the trains in station and Block Section. It functions by

  • detecting situations when two trains are not maintaining a given minimum distance creating a situation for collision, under such a situation
  • it applies brakes
  • Door Drishti (Distant vision) of 3 Kilometers under all weather conditions
  • covering Loco Pilots timely inaction, and v) extending to level crossing gates.

The system is provided in Locos, SLRs for Guard, Level crossing gates, and stations. The trains are tracked by Global Positioning System (GPS) via loco where the ACD is provided, which is satellite based. The loco is connected with GPS through antenna fixed on its top. The system positions the train by locating the train by its latitude, longitude, date, time and speed of the train. ACD, while interacting with each other through radio communication exchange data and commands with each other, analyses the situation on the real time basis and applies brakes as and when required. In case of failure of the system an audio-visual indication shall be displayed.

Train Actuated Warning Device for level crossing gates (TAWD):

Level crossings are always a cause of concern for Indian Railways, be it on account of road/train accidents or detentions due to heavy road traffic. The situation could be eased out by giving advanced information to the gateman of an approaching train as well as by back locking the gate by the train once the gate is closed and locked. An audiovisual warning by way of a hooter and gate signals for road traffic is also extended to the road users while the gate is scheduled to be and is being closed.

Calling-on signal:

Calling-on signal is a subsidiary signal provided on the signal post on which the main signal is mounted, except that of the last stop signal of the station. The signal is taken Off to lead the train up to the next signal ahead, when the running signal on the post of which the Calling-on signal is provided, fails to come Off for any reason whatsoever. Taking Off of Calling-on signal authorizes the Loco Pilot to lead the train at very cautious speed, prepared to stop short of any obstruction on its way, on a track which may already have been occupied. Taking Off of this signal shall not require overlap to be kept clear beyond next signal.

Shunt Signal:

Shunting is the process of organizing coaches or wagons or locomotives for stabling, arranging orientation of the train rack, attaching or detaching any coach, wagon or loco to or from any train. Shunt Signal is meant for hunting within the station section or in block section siding. Shunt Signal may be mounted on a post by itself or on the post of a running signal. Shunt signal on the post by itself shall have On as well as off aspects while when mounted on the post of running signal, it shall have Off aspect only. The domain of each Shunt signal is always up to the next signal be it running or Shunt Signal, up to which it is leading.