Track circuit
This article is about an electrical device ud on railways.For racecours,e race track .
A track circuit is a simple electrical device ud to
de-Illustration of track circuit invented by William Robinson in 1872
tect the abnce of a train on rail tracks ,ud to inform signallers and control relevant signals.
1Principles and operation
The basic principle behind the track circuit lies in the connection of the two rails by the wheels and axle of locomotives and rolling stock to short out an electrical cir-cuit.This circuit is monitored by electrical equipment to detect the abnce of the trains.Since this is a safety ap-pliance,fail-safe operation is crucial;therefore the circuit is designed to indicate the prence of a train when fail-ures occur.On the other hand,fal occupancy readings are disruptive to railway operations and are to be mini-mized.
Track circuits allow railway signalling systems to oper-ate mi-automatically,by displaying signals for trains to slow down or stop in the prence of occupied track ahead of them.They help prevent dispatchers and operators from causing accidents,both by informing them of track occupancy and by preventing signals from displaying un-safe indications.
1.1The basic circuit
A track circuit typically has power applied to each rail and a relay coil wired across them.When no train is prent,the relay is energid by the current flowing from the power source through the rails.When a train is prent,its axles short (shunt )the rails together;the current to the track relay coil drops,and it is de-energid.Circuits
Schematic drawing of track circuit for unoccupied block (ries resistor next to battery not shown)
Schematic drawing of occupied track circuit (ries resistor next to battery not shown)
through the relay contacts therefore report whether or not the track is occupied.
Each circuit detects a defined ction of track,such as a block .The ctions are parated by insulated joints ,usually in both rails.To prevent one circuit from fally powering another in the event
of insulation failure,the electrical polarity is usually reverd from ction to c-tion.Circuits are powered at low voltages (1.5to 12V DC)to protect against line power failures.The relays and the power supply are attached to opposite ends of the c-tion to prevent broken rails from electrically isolating part of the track from the circuit.A ries resistor limits the current when the track circuit is short-circuited.
1.2Circuits under electrification
In some railway electrification schemes,one or both of the running rails are ud to carry the return current.This prevents u of the basic DC track circuit becau
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21PRINCIPLES AND OPERATION
the substantial traction currents overwhelm the very small track circuit currents.
Where DC traction is ud on the running line or on tracks in clo proximity then DC track circuits cannot be ud, similarly if50Hz AC electrification is ud then50Hz AC track circuits cannot be ud.
To accommodate this,AC track circuits u alternating current signals instead of direct current(DC)but typi-cally,the AC frequency is in the range of audio frequen-cies,from91Hz up to10kHz.The relays are arranged to detect the lected frequency and to ignore DC and AC traction frequency signals.Again,failsafe principles dictate that the relay interprets the prence of the signal as unoccupied track,whereas a lack of a signal indicates the prence of a train.The AC signal can be coded and locomotives equipped with inductive pickups to create a cab signalling system.
There are two common approaches to provide a continu-ous path for traction current that spans multiple track cir-cuit blocks.The simplest method installs insulated track circuit joints on only one of the two rails with the cond being a path for the return current and a ground for the track circuit rail.This has the disadvantage of only being able to detect breaks in one rail so the more popular two rail system us impedance bonds to permit traction cur-rent to pass between isolated track circuit blocks while blocking current at track circuit frequencies.
AC circuits are sometimes ud in areas where conditions introduce stray currents,which interfere with DC track circuits.
In some countries,AC-immune DC track circuits are ud on AC electrified lines.This is the predomin
ant method of track circuiting on overhead electrified parts of the UK rail network.One method provides5V DC to the rails,one of the rails being the traction return and the other being the signal rail.When a relay is energid and attached to the track,normal voltage is5V DC.When there is a break in the circuit and there is no train,the volt-age ris to9V DC which provides a very good means for faultfinding.This systemfilters out the voltage induced in the rails from the overhead lines.The track circuits are limited in length to about300m.
1.3Jointless track circuits
Modern track is often continuously welded,the joints be-ing welded during installation.This offers many benefits to all but the signalling system,which no longer has nat-ural breaks in the rail to form the block ctions.The only method to form discrete blocks in this scenario is to u different audio frequencies(AF)in each block c-tion.To prevent the audio signal from one ction passing into an adjacent ction,pairs of simple tuned circuit are connected across the rails at the ction boundary.The tuned circuit often incorporates the circuit to either apply the transmitted signal to the track or recover the received signal from the other end of the ction.
Consider a railway with two block ctions as in the dia-gram.Section1has frequency A injected at th
e left-hand end and received at the right-hand end.Section2contin-ues from the right hand end of ction1where frequency B is injected and then received at the right-hand end of ction
2.
Track circuited railway with two block ctions
There is often a gap between where frequency A is re-ceived and frequency B is injected.This is referred to as a'tuned zone'and is a ction of track where the ampli-tude of frequency A reduces in the direction of ction2 and the amplitude of frequency B reduces in the direction of ction1.The tuned zone can be of the order of20m long.
Advantages of jointless track circuits:•Eliminates insulated block joints,a component li-able to mechanical failure(both of insulation and by introducing stress to adjoining rails)and mainte-nance.
•In electrified areas,jointless track circuits require fewer impedance bonds than any other double rail traction return track circuits.
Disadvantages of jointless track circuits:•Restrictions on placing impedance bonds,hence any connection for electrification purpos,in or near tuned zones as this may upt thefilter properties of the tuned zone.
•Electronic circuits are more vulnerable to lightning strikes.
1.5DC Coded track circuits3
1.3.1Some popular AF track circuits ud in the
UK
Some of the earliest audio frequency track circuits still in u today were made by the Aster company in France. Frequencies of the Aster SF15type track circuit are1700 Hz and2300Hz on one track and2000Hz and2600Hz on the other.SF stands for'single frequency'and was the name given to the units made under licence by ML Engineering in Plymouth,UK.The frequencies are by definition unmodulated.The lack of modulation can lead to availability problems as well as making the signalling safety ca difficult to produce.To address the prob-lems,modulated track circuits were developed like the TI21system.
TI21type track circuits(now known as EBI Track200) u eight nominal frequencies,from1549Hz to2593 Hz for main line applications and eight frequencies from 5600Hz to8400Hz for metro applications(designated TI21-M or EBI Track300).Actual transmission is±17 Hz around the nominal frequency for main line and±100 Hz for metro.The signal is FSK modulated at4.8Hz(20 Hz for metro)unless overridden by the MOD terminal on the front panel.TI stands for'traction immune'and w
as the name ud by ML Engineering in Plymouth.ML Engineering was taken over by various companies and is owned by Bombardier Transportation(2009).TI21main line track circuits can be up to1100m in length.This can be extended to2200m with compensating capacitors. To simplify traction pack design in locomotives,many track circuit manufacturers now transmit a unique code from the transmitter to the receiver.This offers im-provements in availability,simplification in signalling sys-tem design and more robust safety cas.Such systems include the Siemens FTG S,Westinghou(Invensys) FS3000,Bombardier EBI Track400and Alstom’s Digi-code and Jade.
1.4CSEE UM71
CSEE are another kind of jointless track circuit.It us 1700Hz and2300Hz on one track and2000Hz and2600 Hz on the other.[1]To reduce the chance of stray cur-rents causing a wrong side failure the basic frequencies are modulated±15Hz or so.Different rates of modula-tion can be detected by equipment on the trains and ud for ATC,so long as the transmitter end(Tx)is at the front of the train.
The TI21and Westinghou FS2500jointless track cir-cuits are similar to the UM71.
1.4.1Data Pickup Unit
A jointless track circuit such as the CSEE can be divided with a Data Pickup Unit(DPU),which is cheaper than splitting it into two track circuits.A DPU avoids the
need Data Pickup Unit CSEE;end view
to change the frequency of a whole ries of track circuits in a cascade.The DPU consists of a tuned coil which detects the prence or abnce of current in the adjacent rail and picks up or drops a relay accordingly.One u of DPUs is for timing circuits.Each track circuit frequency has its own DPU tuned to that frequency.DPUs can be located almost anywhere;they overcome the limitation that Jointless tracks have a minimum length.
The UM71DPU made by CSEE is triangular while the FS2500DPU made by Westinghou is rectangular. 1.5DC Coded track circuits
In non-electrified areas,DC coded track circuits may be ud.The modulate the current going from the pow-ersource end to the relay end and control the signals and cab signals without the need for line wires.The modu-lated currents can be detected by equipment on the train to provide cab signalling.[2]They can be overlain by pre-dictor systems to operate level crossings.[3]
Brands of coded track circuit include:
•Microtrax by Union Switch&Signal.•Invensys PSO4000[4]
1.6Cut tracks
Where the length of a ction exceeds the practical length of a track circuit,cut tracks can be provided.With a cut track,the relay of the last track cuts the powersource feed of the cond last track circuit,and so on.Cut tracks are only suitable for unidirectional tracks.
Track circuits with ballast contamination will be shorter than tho with good ballast,thus needing more cut tracks.
42FAILURE MODES AND PREVENTION
2Failure modes and prevention 2.1Wheels and brakes
Railway wheels are made from steel and provide a good short circuit from rail to rail(shunt resistance).
Longer trains with more wheels have better conduc-tivity.Short trains or single engines can be a prob-lem.Trains with a single Budd railmotor,which are also lightweight,and with discbrakes,had some problems when they stopped,and had to make a double stop to en-sure good contact with the rails.
Cast iron brake shoes tend to clean the wheels of non-conductive debris(such as leaves and sand-b
ad traction compounds),while disc brakes do not.As a result,some disc-braked vehicles have“scrubber pads”cleaning the wheels to aid in proper track circuit operation.
2.2Relays
Track circuit relays,referred to by signal maintainers as “vital relays,”are specially designed to reduce the chance of wrong-side failures.They may,for example,have carbon-silver contacts to reduce the likelihood of the wrong contacts welding shut after power surges and light-ning strikes.
2.3Circuit failures
The circuit is designed so that most failures will cau a “track occupied”or Track Occupancy Light(TOL)indi-cation(known as a“Right Side”failure in the UK).For example:
•A broken rail or wire will break the circuit between the power supply and the relay,de-energizing the re-lay.See exception below.
•A failure in the power supply will de-energize the relay.
•A short across the rails or between adjacent track ctions will de-energize the relay.
On the other hand,failure modes which prevent the circuit from detecting trains(known as a“Wrong Side”failure in the UK)are possible.Examples include:•Mechanical failure of the relay,causing the relay to be stuck in the“track clear”position even when the track is occupied.
•One perspex ca warped in the heat,and
touched the relay contacts,holding them up.
•Another relay saw a metal washer slip offand
jam the relay contacts up;the half-washers had
to be replaced by full-circle washers.
•Conditions which partially or completely insulates the wheels from the rail,such as rust,sand,or dry leaves on the rails.This is also known as“poor shunting”(“failure to shunt”in North America and Australia).
•Conditions in the trackbed(roadbed)which create stray electrical signals,such as muddy ballast(which can generate a“battery effect”)or parasitic electrical currents from nearby power transmission lines.
•Parasitic oscillations in the equipment that controls the track circuits.[5]
•Equipment which is not heavy enough to make good electrical contact(shunt failure)or who wheels must be electrically insulated.
•A rail break between the insulated rail joint and the track circuit feed wiring would not be detected. Failure modes that result in an incorrect“track clear”sig-nal(known usually in the US as a“fal clear”)may allow a train to enter an occupied block,creating the risk of a collision.Wheel scale and short trains may also be a prob-lem.They may also cau the warning systems at a grade crossing to fail to activate.This is why in UK practice,a treadle is also ud in the circuitry.
Different means are ud to respond to the types of failures.For example,the relays are designed to a very high level of reliability.In areas with electrical problems, different types of track circuits may be ud which are less susceptible to interference.Speeds may be restricted when and where fallen leaves are an issue.Traffic may be embargoed in order to let equipment pass which does not reliably shunt the rails.
Sabotage is possible.In the1995Palo Verde derailment, saboteurs electrically connected ctions of rail which they had displaced to conceal the breaks in the track they had made.The track circuit there
fore did not detect the breaks,and the engine driver was not given a“Stop”indi-cation.Another form of sabotage,not intended to cau a train crash but merely to make trains stop and slow down unnecessarily in an effort to sabotage an economy or po-tential injuries,is to tie a wire between the2rails,causing
a fal obstruction signal.[6][7]
2.4Railhead contamination and rust
The track circuit relies upon an adequate electrical con-tact between the rail and the wheel;contamination can insulate the one from the other.A common problem is fallen leaves,though there have been cas where crushed incts have also caud detection failures.[8]
A more persistent problem is rust.Usually the railhead is kept clean of rust by the regular passage of trains’wheels. Lines which are not ud regularly can become so rusty
5
as to prevent vehicles being detected;ldom-ud points and crossovers and the extremities of terminal platform lines are also prone to rusting.Measures to overcome this include:
•Depression bars or treadles to detect vehicles;•Stainless steel strips(often zig zag in shape)welded on the railheads;
•High voltage impul track circuits;
•Axle counters over the affected ction;and/or •tunnel sticks whereby a track circuit cannot pickup unless a train is detected in the next track circuit.
2.5Scale
Insulated blockjoints can be bridged by wheel scale in some circumstances causing one or two track circuits to fail.This problem may be reduced by having a pair of blockjoints in ries about4m apart.The short4m c-tion would not itlf be track circuited.[9]
2.6Immunization
Electric locomotives must avoid generating noi in the frequencies that track circuits u.The SNCB Class13 had such problems.
2.7Transitory problems
A short,lightweight and fast train passing over an insu-lated blockjoint may disappear from the departing track circuit before it appears in the arriving track circuit,al-lowing fal clear signals to be given.This problem can be overcome by introducing a time delay of say1–2 conds into the departing track circuit.Electronic track circuits such as the CSEE can easily incorporate such a time delay.
3Transmission of status
Track circuit occupancy status,along with status of other signal and switch related devices,may be integrated with a local control panel as well as a remote rail control cen-tre.If the track circuit contains a relay,it can be con-nected to a device for nding status information via a communications link.The status can then be displayed and stored for archival for purpos of incident investiga-tion and operations-related analysis.Many signalling sys-tems also have local event recorders for recording track circuit status.4Siding turnout
It is sometimes convenient to wire the detectors of a t of points through the track circuit over tho points.This can be done in one of two ways:
•a contact of the points detector can shunt the track circuit when the points are rever,putting the si
g-nals to red,however this is not failsafe.
•the track circuit can be split with extra blockjoints and the detectors in the points complete the track circuit when the points are normal and the signal is entitled to receive a green light.This is partially fail-safe.
•a cond relay can be installed on the turnout,with its contacts wired in ries with the main relay.This is fail-safe but expensive.
5Track circuit clips
A simple piece of safety equipment that can be carried by trains is a track-circuit clip.This is simply a length of wire connecting two metal spring clips that will clip onto a rail.In ca of accident or obstruction a clip applied to a track will indicate that that track is occupied,there-fore putting signals to danger.As an example of u,if a train is derailed on a double track,and is foul of the c-ond track,application of a clip to the cond track will immediately return signals protecting the cond track to danger.This procedure is a much more effective safety measure than attempting to contact a signalling centre by telephone becau its effect is immediate and automatic. 6History
Thefirst u of track circuiting was by William Robert Sykes on a short stretch of track of the London Chatham and Dover Railway at Brixton in1864.[10]The failsafe track circuit was invented in1872by William Robinson, an American electrical and mechanical engineer.His in-troduction of a trustworthy method of block occupancy detection was key to the development of the automatic signalling systems now in nearly universal u.[11]:3ffThefirst railway signals were manually operated by signal tenders or station agents.When to change the signal as-pect was often left to the judgement of the operator.Hu-man error or inattentiveness occasionally resulted in im-proper signalling and train collisions.
The introduction of the telegraph during the mid-nineteenth century showed that information could be electrically conveyed over considerable distance,spurring the investigation into methods of electrically controlling railway signals.Although veral systems were developed
