A
track circuit is a simple electrical device used to detect the presence or absence of a
trainA train is a connected series of vehicles for rail transport that move along a track to transport freight or passengers from one place to another. The track usually consists of two rails, but might also be a monorail or maglev guideway....
on
rail tracksRail tracks are the surface structures that support and guide trains or other rail-guided transportation vehicles.Most familiarly they consist of* two steel rails on which the vehicle wheels run,...
, used to inform signallers and control relevant signals.
Principles and operation
The basic principle behind the track circuit lies in the connection of the two rails by the wheels and axle of
locomotiveA locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...
s and
rolling stockRolling stock is the collective term that describes all the vehicles that move on a railway. It usually includes both powered and unpowered vehicles, for example locomotives, railroad cars, coaches and wagons...
to short out an electrical circuit. This circuit is monitored by electrical equipment to detect the presence or absence of the trains. Since this is a safety appliance,
fail-safeFail-safe or fail-secure describes a device or feature which, in the event of failure, responds in a way that will cause no harm or at least a minimum of harm to other devices or danger to personnel...
operation is crucial; therefore the
circuit is designed to indicate the presence of a train when failures occur. On the other hand, false occupancy readings are disruptive to railroad operations and are to be minimized.
Track circuits allow
railway signallingRailway signalling is a system used to control railway traffic safely, essentially to prevent trains from colliding. Being guided by fixed rails, trains are uniquely susceptible to collision; furthermore, trains cannot stop quickly, and frequently operate at speeds that do not enable them to stop...
systems to operate semi-automatically, by displaying signals for trains to slow down or stop in the presence 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 unsafe indications.
The basic circuit
A track circuit typically has power applied to each rail and a
relayA relay is an electrically operated switch. Electric current through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts...
coil wired across them. Each circuit detects a defined section of track, such as a block. These sections are separated by insulated joints, usually in both rails. To prevent one circuit from falsely powering another in the event of insulation failure, the electrical
polarityIn physics, polarity is a description of an attribute, typically a binary attribute , or a vector . For example:* An electric charge has a polarity of either positive or negative....
is usually reversed from section to section. Circuits are commonly battery-powered at low voltages (1.5 to 12 V DC) to protect against line power failures. The relays and the power supply are attached to opposite ends of the section in order to prevent broken rails from electrically isolating part of the track from the circuit.
When no train is present, the relay is energised by the current flowing from the power source through the rails. When a train is present, its axles short (
shuntIn electronics, a shunt is a device which allows electric current to pass around another point in the circuit. The term is also widely used in photovoltaics to describe an unwanted short circuit between the front and back surface contacts of a solar cell, usually caused by wafer damage.-Defective...
) the rails together; the current to the track relay coil drops, and it is de-energised. Circuits through the relay contacts therefore report whether or not the track is occupied.
A series resistor limits the current when the track circuit is short circuited, saving battery power.
Circuits under electrification
In almost all railway electrification schemes one or both of the rails are used to carry the return current. This prevents use of the basic DC track circuit because the substantial traction currents overwhelm the very small track signal currents.
To accommodate this,
AC track circuits use alternating current signals instead of DC currents. Typically, the AC frequency is in the range of audio frequencies, from 91 Hz up to a 250 Hz. The relays are arranged to detect the selected frequency and to ignore DC and AC traction frequency signals. Again, fail safe principles dictate that the relay interprets the presence of the signal as unoccupied track, whereas a lack of a signal indicates the presence of a train. The AC signal can be coded and locomotives equipped with inductive pickups to create a
cab signallingCab signalling is a railroad safety system that communicates track status information to the train cab , where the engineer or driver can see the information...
system.
In this system, impedance bonds are used to connect items which must be electrically connected for electrification purposes but which must remain isolated to track circuit frequencies for the track circuit to function.
AC circuits are sometimes used in areas where conditions introduce stray currents which interfere with DC track circuits.
In some countries, AC-immune DC track circuits are used on AC electrified lines. One method provides 5V DC to the rails, one of the rails being the traction return and the other being the signal rail. When a relay is energised and attached to the track, normal voltage is 5V DC. When there is a break in the circuit and there is no train, the voltage rises to 9V DC which provides a very good means for fault finding. This system filters out the voltage induced in the rails from the overhead lines.
Jointless track circuits
Jointless track circuits use audio frequency tuned circuits to create what amounts to a block joint to signalling frequency currents and a very low impedance to electrification power frequency currents. The track can be resonated with the tuning components in the track circuit system so as to create a 'pole' at the wanted frequency and 'zero' at the adjacent unwanted frequency.
Frequencies of the Aster SF 15 type track circuit are 1700 Hz and 2300 Hz on one track and 2000 Hz and 2600 Hz 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 original Aster track circuits were made by the Aster company in France. These frequencies are by definition unmodulated.
TI21 type track circuits (now known as
EBI Track 200) use eight nominal frequencies, from 1549 Hz to 2593 Hz for main line applications and eight frequencies from 5600 Hz to 8400 Hz for metro applications (designated TI21-M or
EBI Track 300). Actual transmission is ± 17Hz around the nominal frequency for main line and ±100 Hz for metro. The signal is
FSKFrequency-shift keying is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK . BFSK literally implies using a couple of discrete frequencies to transmit binary information...
modulated at 4.8 Hz (20 Hz for metro) unless overridden by the MOD terminal on the front panel. TI stands for 'traction immune' and was the name used by ML Engineering in Plymouth. ML Engineering was taken over by various companies and is owned by
Bombardier TransportationBombardier Transportation is the rail equipment division of the Bombardier Inc.. Bombardier Transportation is the world’s largest company in the rail equipment manufacturing and servicing industry. Its headquarters is in Berlin, Germany....
(2009). TI21 main line track circuits can be up to 1100m in length. This can be extended to 2200m 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. Such systems include the Siemens FTG S,
WestinghouseWestinghouse Rail Systems Ltd is a British supplier of railway signalling and control equipment to the rail industry worldwide. Its head office is in Chippenham, Wiltshire, where it manufactures a variety of mechanical and electrical/electronic railway signalling equipment...
(
InvensysInvensys plc is a British engineering company, headquartered in London, England. The company is multinational with operations in over 60 countries, employing around 23,000 employees. Invensys is quoted on the London Stock Exchange and is a constituent of the FTSE 100 Index.-History:The company was...
) FS3000 and Bombardier
EBI Track 400.
Coding prevents interference from affecting both the safety and availability of the track circuit.
Advantages of jointless track circuits:
- Eliminates Insulated Block Joints, a component liable to mechanical failure (both of insulation and by introducing stress to adjoining rails) and maintenance.
- 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 purposes, in or near tuned zones as this may upset the filter properties of the tuned zone.
- Electronic circuits are more vulnerable to lightning strikes.
CSEE UM71
CSEE are another kind of jointless track circuit. It uses 1700 Hz and 2300 Hz on one track and 2000 Hz and 2600 Hz on the other. To reduce the chance of stray currents causing a wrong side failure the basic frequencies are modulated ±15 Hz or so. Different rates of modulation can be detected by equipment on the trains and used for
ATCAutomatic Train Control is a train protection system for railways, ensuring the safe and smooth operation of trains on ATC-enabled lines. Its main advantages include making possible the use of cab signalling instead of track-side signals, and the use of smooth deceleration patterns in lieu of the...
.
The
TI21 and Westinghouse
FS2500 jointless track circuits are similar to the UM71.
DPU
A jointless track circuit such as the CSEE can be divided with a
Data Pickup Unit (
DPUDPU is a three-letter acronym that can stand for:* in computing: Data Path Unit* in quality management: Defects per Unit.* in Turkish education: Dumlupınar University....
), which is cheaper than splitting it into two track circuits. The DPU consists of a tuned coil which detects the presence or absence of current in the adjacent rail and picks up or drops a relay accordingly. One use of DPUs is for timing circuits. Each track circuit frequency has its own DPU tuned to that frequency.
The DPU made by CSEE is triangular while the FS2500 DPU made by
WestinghouseWestinghouse may refer to:In current companies:*Westinghouse Electric Corporation , and its licensees:**Westinghouse Digital Electronics, selling LCD televisions and related products...
is rectangular.
Wheels and brakes
Railway wheels are made from steel and provide a good short circuit from rail to rail.
The longer the train and the more wheels the better. Short trains or single engines can be a problem. Single Budd railmotors which are lightweight had some problems when they stopped, and had to make a double stop to ensure good contact with the rails.
Cast iron brake blocks tend to clean the wheels of non-conductive debris (for example leaf mulch and sand-based compounds applied to the rails to improve adhesion in icy conditions), while disc brakes do not, as a result some disc braked vehicles have wheel brake like "scrubber pads" in contact with the wheel to keep the wheel clean and free from contaminants which could prevent the track circuit method operating correctly.
Relays
Track circuit
relayA relay is an electrically operated switch. Electric current through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts...
s are specially designed to reduce the chance of wrong side failures. They might for example have carbon-silver contacts to reduce the likelihood of the wrong contacts welding shut after power surges and lightning strikes.
Circuit failures
The circuit is designed so that most failures will cause a "track occupied" indication (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 relay. See exception below.
- A failure in the power supply will de-energize the relay.
- A short across the rails or between adjacent track sections 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.
- 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.
- Equipment which is not heavy enough to make good electrical contact (shunt failure) or whose 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" signal (known usually in the US as a "false 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 problem. They may also cause the warning systems at a grade crossing to fail to activate. This is why in UK practice, a
treadleIn railway signalling, a treadle is a mechanical or electrical device that detects that a train axle has passed a particular location. They are used where a track circuit requires re-inforcing with additional information about a train's location, such as around an automatic level crossing, or in an...
is also used in the circuitry.
Different means are used to respond to these 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 used 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 the
1995 Palo Verde derailmentThe 1995 Palo Verde derailment took place on October 9, 1995, when an Amtrak Sunset Limited train derailed near Palo Verde, Arizona on Southern Pacific Railroad tracks. The two locomotives and eight of the twelve cars derailed, four of them falling 15 feet off a bridge into a dry river bed...
, saboteurs electrically connected sections of rail which they had displaced to conceal the breaks in the track they had made. The track circuit therefore did not detect the breaks, and the engineer was not given a stop indication.
Railhead contamination
For a track circuit to operate reliably, the railheads must be kept clean of rust by the regular passage of trains' wheels. Track circuited lines that are not used regularly can become so rusty as to prevent vehicles being detected. Seldom-used points and crossovers and the extremities of terminal platform lines are prone to rusting. Measures to overcome this include:
- Provision of a depression bar or treadle to detect vehicles;
- Provision of a stainless steel
In metallurgy, stainless steel, also known as inox steel or inox, is defined as a steel alloy with a minimum of 11% chromium content by mass. Stainless steel does not stain, corrode, or rust as easily as ordinary steel...
strip (often zig zag in shape) welded on the railheads;
- The use of a high voltage impulse track circuit such as those made by Jeumont-Schneider;
- The use of axle counters over the affected section.
- Provision of "tunnel sticks" whereby a track circuit cannot pickup unless the train is proven to be occupying the next track circuit.
Another source of railhead contamination is leaf-fall (
see also slippery railSlippery rail is a condition of railroads caused by fallen moist leaves that lie on and cling to the top surface of the rails of railroad tracks. The condition results in significant loss of friction between train wheels and rails, and in extreme cases can render the track temporarily unusable...
).
Transmission 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 centre. If the track circuit contains a relay, it can be connected to device for sending status information via a communications link. The status can then be displayed and stored for archival for purposes of incident investigation and operations-related analysis. Many signalling systems also have local event recorders for recording track circuit status.
Track 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 sprung clips that will clip onto a rail. In case of accident or obstruction a clip applied to a track will indicate that that track is occupied, therefore putting signals to danger. As an example of use, if a train is derailed on a double track, and is foul of the second track, application of a clip to the second track will immediately return signals protecting the second track to danger. This procedure is a much more effective safety measure than attempting to contact a signalling centre by telephone because its effect is immediate and automatic.
History
The
failsafeFail-safe or fail-secure describes a device or feature which, in the event of failure, responds in a way that will cause no harm or at least a minimum of harm to other devices or danger to personnel...
track circuit was invented in 1872 by
William RobinsonWilliam Robinson, or Will Robinson or Bill Robinson or other nicknames, may refer to:-Historical:* William Robinson , Quaker martyr* William Benjamin Robinson , Canadian fur trader and political figure...
, an American
civil engineerA civil engineer is a person who practices civil engineering, one of the many professions of engineering. Originally a civil engineer worked on public works projects and was contrasted with the military engineer, who worked on armaments and defenses...
. His introduction of a trustworthy method of block occupancy detection was key to the development of the automatic signalling systems now in nearly universal use.
The first railway signals were manually operated by
signal tendersA signalman or signaller is an employee of a railway transport network who operates the points and signals from a signal box in order to control the movement of trains.- History :...
or station agents. When to change the signal
aspectAspect may be:*Aspect , a feature that is linked to many parts of a program, but which is not necessarily the primary function of the program...
was often left to the judgement of the operator. Human error or inattentiveness occasionally resulted in improper signalling and train collisions.
The introduction of the
telegraphThe electrical telegraph is a telegraph that uses electric signals. The electromagnetic telegraph is a device for human-to-human transmission of coded text messages over wire...
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 several systems were developed prior to Robinson's, none could automatically respond to train movements.
Robinson first demonstrated a fully automatic railway signalling system in
modelA physical model is a smaller or larger physical copy of an object. The object being modelled may be small or large ....
form in 1870. A full-sized version was subsequently installed on the
Philadelphia and Erie RailroadThe Philadelphia and Erie Railroad was originally called the Sunbury and Erie Railroad. The name was changed in 1861 by the state of Pennsylvania to encourage its completion from Sunbury, Pennsylvania to Erie, Pennsylvania, which was finished in 1864. Never a prosperous line, it was shortly after...
at Ludlow, Pennsylvania (aka Kinzua, PA), where it proved to be practical. His design consisted of electrically operated discs located atop small trackside signal huts, and was based on an open track circuit. When no train was within the block no power was applied to the signal, indicating a clear track.
An inherent weakness of this arrangement was that it could fail in an unsafe state. For example, a broken wire in the track circuit would falsely indicate that no train was in the block, even if one was. Recognizing this, Robinson devised the closed loop track circuit described above, and in 1872 [BrMc81, Ph93], installed it in place of the previous circuit. The result was a fully automatic, failsafe signalling system that was the prototype for subsequent development.
Although a pioneer in the use of signals controlling trains, the
United KingdomThe United Kingdom of Great Britain and Northern Ireland is a sovereign state located off the northwestern coast of continental Europe. It is an island country, spanning an archipelago including Great Britain, the northeastern part of Ireland, and many small islands...
was slow to adopt Robinson's design. At the time, many carriages on
UKThe United Kingdom of Great Britain and Northern Ireland is a sovereign state located off the northwestern coast of continental Europe. It is an island country, spanning an archipelago including Great Britain, the northeastern part of Ireland, and many small islands...
railways had wooden
axleAn axle is a central shaft for a rotating wheel or gear. In some cases the axle may be fixed in position with a bearing or bushing sitting inside the hole in the wheel or gear to allow the wheel or gear to rotate around the axle. In other cases the wheel or gear may be fixed to the axle, with...
s and/or
wheelA wheel is a circular device that is capable of rotating on its axis, facilitating movement or transportation whilst supporting a load , or performing labour in machines. Common examples are found in transport applications. A wheel, together with an axle overcomes friction by facilitating motion by...
s with wooden
hubHub means the center of a wheel. By analogy, it generally means 'center' or 'important place', and may refer to: -Technical meanings:* Wheel hub motor, electric motor incorporated into a hub of a wheel* Bicycle hub, the central part of a bicycle wheel...
s, making them incompatible with track circuits.
Caused by lack of track circuits
Numerous accidents would have been prevented by the provision of track circuits, including:
Norton Fitzwarren rail crash (1890)The Norton Fitzwarren rail crash occurred on 11 November 1890, at Norton Fitzwarren station on the Great Western Railway, approximately two miles south-west of Taunton in Somerset. A special boat train carrying passengers from Plymouth to Paddington collided with a goods train that was being...
Quintinshill rail crashThe Quintinshill rail disaster occurred on 22 May 1915, at Quintinshill, an intermediate signal box with refuge loops on the Caledonian Railway Main Line near Gretna Green in Scotland. Involving five trains, the crash killed 226* people and caused by far the most casualties of any rail crash that...
Hawes Junction rail crashThe Hawes Junction rail crash occurred on 24 December 1910, on the Midland Railway's Settle and Carlisle mainline at the junction with the Wensleydale Railway in Westmorland , England. It was caused when a busy signalman forgot about a pair of light engines waiting at his down starting signal to...
Geurie crossing loop collisionThe Geurie crossing loop collision occurred on the night of 23 August, 1963. Geurie is located between Orange, New South Wales and Dubbo.- Events leading up to the Accident :...
- 1963
Redfern - 1894 - incorrectly set signal
Caused by track circuit failure
Much rarer are accidents caused when the track circuits themselves fail. For example: Cowan rail disaster, which occurred when sand on the rails insulated the wheels from the rails, causing a failure to shunt that allowed a trailing
block signalA signal is a mechanical or electrical device erected beside a railway line to pass information relating to the state of the line ahead to train drivers/engineers. The driver interprets the signal's indication and acts accordingly...
to improperly display a clear aspect, resulting in a rear end collision.
Broken rails
Track circuits can detect some but not all broken rails. If there is only one relay, part of the wiring of a turnout is bound to be in parallel, which is not protected from broken rails. If there are two relays, then all parts of the wiring can be in series, protecting against broken rails. Thus:
Weyauwega derailmentThe Weyauwega derailment was a railroad accident that occurred in Weyauwega, Wisconsin, USA, in the early morning hours of March 4 1996. The derailed train was carrying a large quantity of hazardous material, which immediately caught fire...
- undetected broken rail in turnout.