Track Circuits (2025)

Question 1

What is a purpose of a track circuit?

Answer 1

To detect the absence of a Train.

Question 2

At what voltage is the A.C. Main supplied?

Answer 2

600v A.C.

Question 3

What frequencies is the A.C. Main supplied at and what colours would the track relays be in these areas?

Answer 3

Red: 33 1/3 Hz
Silver 125 Hz

Question 4

When would it be allowable to work live on the AC main?

Answer 4

Under NO circumstances.

Question 5

What secondary output voltages are available from a trackside transformer?

a) trackwinding
B) 7 way bay winding

Answer 5

A) 125v
B) 110v

Question 6

Why is BX selection a requirement of signalling circuits fed from the 7 way fuse bay?

Answer 6

If an Earth fault was to develop on the BX leg of the signal selection the fuse would blow due to excess current and bypassing the load (TR) which would be a self-announcing failure.
If one Earth was to develop on the NX leg it would go unnoticed until it was found during maintenance activities known as a latent failure.
If the busbar was not earthed at the 7 way fuse bay and two or more Earth faults were to develop on the BX leg it could bypass safety critical contacts which would result in an wrong-side failure.

Question 7

Why are there 2 pigtails connecting the track wire to the rail?

Answer 7

Redundancy.

Question 8

What size of cable is track wire?

Answer 8

0.85mm Track Wire.

Question 9

What are the three main functions of a capacitor within a track circuit?

Answer 9

Choose 3 of the following:
1. Acts as a load when the train wheels shunt the track.
2. A means of adjusting current.
3. Creates a 90 degrees phase relation for optimal relay performance.
4. Only allows A.C. to pass through it.

Question 10

How far into the green area of the Vane Indicator should the Vane Pointer of the TR be if the track circuit is half outdoors and half in a tunnel?

Answer 10

2/3

Question 11

How many bonds are required over a non-insulated rail joint on the continuous rail? Why is this?

Answer 11

2. For capacity in case of traction fault conditions from a metallic object shorting from the positive traction rail to the continuous rail.

Question 12

What is the maximum capacitance a TO is authorised to put into a track circuit?

a) 33 1/3 Hz
b) 125 Hz

Answer 12

A) 33 1/3 Hz = 40 micro farads.
B) 125 Hz = 10 micro farads.

Question 13

What are the different types of floating failures? And how are they tested?

Answer 13

A) Lack of capacitance
- Clip + Lift = 100v
- Output of cap = Low Volts
- Increase capacitance

B) Partial Short or Earth
- Clip + Lift = Low Volts
- Find with current clamp

C) High Resistance in Series
- Clip + Lift = 100v
- Output of Cap = High Volts
- Find with volt meter

Question 14

What action may be taken to increase current flow in the circuit without overloading the trackside transformer?

Answer 14

Change link at transformer from t0 - t1 to t0 - t2
100v to 125v

Question 15

What documentation is required prior to any adjustment of the track circuit, and what tests or checks must be carried out following any adjustments?

Answer 15

AWC + Permission from manager.
1 ohm shunt track at extremities + mid, 1m of points.
Check relay energisation. 1/3 tunnel, 2/3 outdoor.

Question 16

What must be done when track conditions change?

Answer 16

1 ohm Shunt track at extremities + mid.
Check relay energisation. 1/3 Tunnel, 2/3 Outdoor.

Question 17

Why is the R coil of the TR wired in series with an R coil of a second TR? How else could this additional protection be achieved?

Answer 17

To achieve berth track diversity. This adds an extra layer of protection when the train is at its most vulnerable position. TR-TR protects the GR from energising when one of the TRs becomes mechanically stuck. Two TRs for the same track circuit will be in the signal selection of the GR so if one becomes mechanically stuck up the other will still drop out as both are needed to feed the GR.

TR-GR. This only works if the GR is a DEV. The GR will still have a feed for the Q coil via its signal selection but also has feed for its R-coil from the berth track circuit form the signal in front. If the TR was to become mechanically stuck up the GR would still be de-energised as it is fed from
the same track circuit for its R-coil.

Question 18

What does “fail safe” mean?

Answer 18

When the signalling system fails but is left in a safe state. For example the TR would drop in the event of an IBJ failure due to opposition. Also known as a right-side failure.

Question 19

What is an example of a wrong side failure?

Answer 19

When a TR picks up underneath a train. Turning the signal in the rear from
danger to proceed when there is a train in the signal’s L.O.C.

Question 20

Why is a 1:1 isolating transformer required when track circuits are fed from busbars in a relay room?

Answer 20

Busbars in a relay room are earthed. Track circuits require an earth free supply as they already have a known earth (continuous rail). The 1:1 isolating transformer removes the earth from the supply.

Question 21

List 5 pre-instillation checks you would undertake before replacing a D.E.V. Track Relay.

Answer 21

Any 5 of the following:

1. Like-for-like
   - Double Element Vane
   -Track Relay/Contact Arrangement
   -Correct frequency
2. Continuity checks on armatures.
3. Overhauled within last 3 years.
4. Part No and Serial No
5. Check for signs of damage
   -Spindle moves freely and is not bent.
   -Glass is intact.
   -No loose internal wires.
6. Impact detector is not dislodged.

Question 22

Why is track circuit opposition employed within capacitor fed track circuits?

Answer 22

In the event of an IBJ failure at least 1 track circuit will fail. If opposition was not achieved the track circuit could potentially become one big track circuit this is especially dangerous between a berth track and replacing track as one a train hits a berth track it could change the replacing track to danger at the same time therefore raising the trainstop and causing an high speed SPAD.

Question 23

What is the electrical characteristic of an anti-opposition track?

Answer 23

No phase relationship to adjacent track circuits.
Jumper rail cables.
IBJs on either side of the rail.

Question 24

List three places and their reason why you would find an anti-opposition track.

Answer 24

A) Transposing of +ve traction rail.
- Continuous rail must always be next to +ve traction rail for dissipation of traction fault current.

B) Substation Boundary
- Mitigate against phase drift.

C) Platform Areas
- Maintainers break.

Question 25

List three failing conditions that would leave T1R UP and T2R DOWN.

Answer 25

1. Open circuit in T2R Q-coil. 2. Earth in the mid potential. 3. Earth fault at R3 T2R.

Question 26

How is Automatic Train Protection provided in a conventionally signalled area?

Answer 26

In the form of trainstops. A trains tripcock will engage with the trainstop if the train goes pass a signal when at danger which will apply the trains emergency brake.

Question 27

What unsafe condition could potentially occur if a fan track was bonded incorrectly?

Answer 27

If a fan track was bonded incorrectly and was wired in parallel instead of series a train could not be clear of the track circuit and another train could be signalled into the area and cause a collision with the rear of the train already in the section as the relay could energise. If only one bond was applied and it fell off it would cause the relay to drop and the signal being unable to clear a second bond provides redundancy allowing the service to keep following. If the bonds are not applied at the of each section and at the mid point or start a train could not potentially be fully clear of the fan track before another train is signalled in as the track relay may have picked up causing a collision.

Question 28

Why are track circuit interrupters installed on the railway?

Answer 28

They are installed in case a train enters an illegal zone. For example if a train was to go to far pasts its stopping diamond and hit a FRL it would knock the TCI not allowing the track to pick up as the TCI forms part of the track circuit. When the train leaves it won’t allow another train to signal into the area as when the last train knocked over the FRL the next train might not see where the end of line is and possibly derail into the sand drag. Another purpose of the TCI is if a train went into a sand drag the TCI would have been knocked and not allowing another train into the area which could lead to a wreck on a wreck.

Question 29

What would be the possible outcome if the TCI was installed in the incorrect position after a rerail? (ie within 3 meters of the FRLs)

Answer 29

The front of a train is 3m in front of the first set of axels if a TCI was placed within 3m it is possible the front of a train could hit the FRL and knock it down without hitting the TCI therefore making it impossible to know that a FRL was hit. Once the train leaves another train train can be signalled into the area and wouldn’t see the FRL as it was knocked down and it is possible that it could hit the sand drag/train arrestor causing damage to the rolling stock.

Question 30

List three places where check rails may be used.

Answer 30

1. Adjacent to tripcock testers. 2. Points and crossing. 3. Tight curvature around bends.

Question 31

How does the system protect against the consequences of a berth track relay becoming mechanically stuck up?

Answer 31

By incorporating berth diversity. One way this is done is by TR-TR which requires both relays within the signal selection of the GR to be energised to pick up the GR. If one of these are de-energised it is impossible for the GR to pick up as they are wired in series. TR-GR this requires the TR within the Q-side signal selection to pick up feeding the GR and for the R-coil to pick up which is wired in the berth track of the signal in front. Both R and Q are needed to pick up the GR.

Question 32

What effect could rusty or contaminated rails have on a track circuit?

Question 33

What is the effect of an Earth fault developing between 1F 315TR and 1 ON contact 317V?

Answer 33

The fuse would blow instantly due to excess current due to bypassing the load (TR). This would lead to the TR de-energising and the signal turning to danger as it’s lost the signal selection path in the Q side of the GR.

Question 34

What are the fundamental principles of signalling?

Answer 34

Keeping a safe separation of trains at all times and safeguarding the movement of trains through points and crossings.

Question 35

What testing is required when installing the additional transformer into the circuit?

Answer 35

Receiving an AWC and testing meter on a known source. Isolate floating track. Connect a wire from t0 - t0 and insert fuses for both. Place volt meter on t1 - t1. If you read 0v replace meter with wire as this means it is in parallel and in phase. If you read 200v it is out of phase and need to move first wire from t0 - t1. Retest and if you get 0v it is in phase.

Question 36

Why is a 1:1 transformer required when connecting a transformer from the 7 way fuse bay?

Answer 36

Because the 7 way fuse bay is earthed. Our track circuits require an earth free supply as we already have a known earth in our circuit (continuous rail). A 1:1 isolating transformer removed the earth from the supply. If we did not include an isolating transformer our TR would never pick up.

Question 37

What checks are now required after adjusting capacitance on a track circuit?

Answer 37

1 ohm shunt at extremities and mid, every 1m over points. Check relay energisation. 1/3 Tunnel and 2/3 outdoor.

Question 38

Which engineering standard provides guidance on track circuit maintenance?

Answer 38

S2541

Question 39

Which engineering standard provides guidance on irregular connections/adjustments?

Answer 39

S2530

Question 40

What action will you take when finding a wire degradation with green spots?

Answer 40

Stop work and take a picture of the wire degradation. Escalate to DSIM straight away with evidence. Inspect the rest of the cable for full extent of issue. Straps/other polymers such as cable ties can speed up the process. Do not touch as it can be conductive and has the potential to bridge two or more wires bypassing safety critical contacts. Send pictures/evidence to material engineer to confirm what it is and ensure it is sleeved before continuing any works. Once achieved take pictures are send to DSIM.

Question 41

A Siemens wheel detector sensor has been temporarily removed from the rail due to the re-rail. How should it be remounted (in relation to the crown of the rail) and what are the minimum tests due to be carried out?

Answer 41

The Siemens wheel detector should be re-mounted in the exact same location as where it was removed as circuitry that includes route exit and speed control are calculated at that exact position. It should be exactly 41mm +/- 1mm from the crown of the rail. Place a dummy wheel on the track and observe evaluator card, when the wheel is on the light should go out and when it is removed the light should be on, this process simulates a train on the track.

Question 42

An Earth fault in the track local coil occurs. a) What is the immediate effect? (2 marks) b) How will you determine the cause of the fault? (2 marks) c) How will you locate the fault? (2 marks)

Answer 42

A) The fuse would blow due to excess current as it would bypass the load (TR). The relay in the rear would de-energise at it has lost its Q-Coil. Changing the singer from proceed to danger. B) Replace fuse (blows again). With AWC + Manager permission remove the NX Earth wire from the circuit and replace the fuse, if it blows it’s and Earth fault if it doesn’t it’s a short. C) Make disconnections within the circuit to locate fault after each disconnection replace fuse/MCB and if it blows it’s still in the circuit if it doesn’t you have disconnected the Earth so inspect the part you disconnected. This can be done by lifting Q1.

Question 43

What is the purpose of TQ proving circuit?

Answer 43

Trainstop proving circuit ensures that the trainstop comes into the ‘ON’ position after the passage of every train. TQ proving circuits are found on replacing tracks track circuits and ensure that the rear of a train is always protected by at least one trainstop (ATP) in the event where a trainstop is not detected in the ‘ON’ position. The signal will stay at danger. It has a stick and pick path, stick acts as the memory that the condition happened and the pick accounts for changes in the circuit, both need to be met in order to change the signal to proceed.