Signalling 

Infrastructure Risk Models

Electric Machine Points Operating System HW Type 

Hydraulic Machine Point Operating System - Clamplock Type 

Hand Point Operating System, Lever Type 

Jointed Track Circuits Train Detection DC 9 OHM Relay Type & 
Jointless Track Circuits Train Detection T1 21 Type 

TCAID Train Detection 

Axle Counter Train Detection SEL Type 

Mechanical Presence Detector Treadle Type 

Automatic Warning System Standard Power Type 

Automatic Train Protection System Great Western Type 

Geographical Interlocking 

Absolute Block Interlocking 

Solid State Interlocking SSI Type 

Mechanical Signals Main/Subsidiary Signal, Upper Quadrant Type 

Electrical Signals - Multiple Aspect Main Type 

Junction Indicator Signals Position Light Type 

Automatic Level Crossing, Automatic Half Barrier Type 

Manual Level Crossing,, Manually Controlled Barrier with CCTV Type 

HVI Track Circuits 

Reed Track Circuits

Electric Machine Points Operating System HW Type 

Electrical points provide a powered operating system for points, normally operated remotely from the signal box.

The points are driven electro-mechanically and move over a lubricated slide chair which ensures that the switch rails move freely when operated.

Micro-switches are incorporated into the detection unit to provide point detection, ensuring that the points are set correctly before the preceding signal clears.

Hydraulic Machine Point Operating System - Clamplock Type 

Hydraulic points provide a powered operating system for points, normally operated remotely from the signal box.

The points are driven by Hydraulic Actuators, powered by a pump, which has an interlocked hand pump to provide emergency manual back up. The points move over a lubricated slide chair which ensured that the switch rails move freely when operated.

Clamplocks are used to lock the points in position to reduce the risk of derailment due to gauge spread or movement of the point under a train. Micro switches are incorporated into the clamplock unit to provide point detection, this ensures that the points are set correctly before the preceding signal clears.

Hand Point Operating System, Lever Type 

Hand points provide a manual operating system for points where powered systems are not cost effective, for example in depots and sidings.

The points are operated by a lever, connected via the drop box to rodding, which drives the points. The points move over a lubricated slide chair which ensures that the switch rails move freely when operated. No locking or detection are provided, for passenger train movements over hand points the points need to be secured using clips and scotch blocks.

Jointed Track Circuits Train Detection DC 9 OHM Relay Type & 
Jointless Track Circuits Train Detection T1 21 Type 

Track circuits are systems used to determine whether a section of track is occupied by a train. A section of track is bounded at one end by an electrical source and transmitter electronics and at the other by receiver electronics and a relay. When the section of track is unoccupied the circuit is complete and the relay is energised. When a train enters the section the system is short-circuited and the relay is de-energised.

TCAID Train Detection 

Track circuits are systems used to determine whether a section of track is occupied by a train. A section of track is bounded at on end by an electrical source and transmitter electronics and at the other by receiver electronics and a relay. When the section of track is unoccupied the circuit is complete and the relay is energised. When a train enters the section the system is short-circuited and the relay is de-energised. Failure modes of track circuits were considered in a previous Trident report.

Axle Counter Train Detection SEL Type 

Axle counters are installed as a means of proving that an entire train has left a track section by showing as many axles have come out of the section as entered it.

Mechanical Presence Detector Treadle Type 

Treadles are mechanically operated switches mounted adjacent to the running rail which are activated by the flange of the train wheel as it passes. They are used to maintain information on train position and to activate level crossing barriers.

Automatic Warning System Standard Power Type 

The automatic warning system (AWS) supervises the driver's reaction to restrictive signal aspects. Whilst the system is advisory it causes a brake application to be made automatically if the driver fails to react when approaching a restrictive aspect.

Automatic Train Protection System Great Western Type 

ATP provides a means of protection against errors by train drivers. The key to ATP is speed supervision. After passing a caution signal the train speed is repeatedly checked to compute its braking rate. If this is insufficient to stop the train at the red signal, the speed supervision system can impose a full brake application to bring the train safely to stand. Similar arrangements can ensure that trains brake sufficiently to comply with speed restrictions.

Geographical Interlocking 

Geographical interlocking systems provide a form of interlocking which incorporates the relays into a number of standard factory wired sets. The systems may be programmed for bi-directional working, and are specifically designed to cope with the variations in the controls of main signals, draw-ahead, shunt, ahead subsidiary and shunting signals. This report addressed the failure of the Solid State Interlocking System.

Absolute Block Interlocking 

Absolute Block is a system involving people and technology for the safe working of trains between two points. The technology aids the signaller to follow correct procedures laid down in the Rule Book. It is now largely superseded on the main lines by the track circuit block system. However, the Absolute Block System of Signalling can still be found in certain geographical areas of the Railtrack network such as rural secondary lines.

Solid State Interlocking SSI Type 

The Solid State Interlocking (SSI) System was designed to substitute relay interlocking systems with a safer and more reliable system for co-ordinating the operation of track points and signals. This report addressed the failure of the Solid State Interlocking System.

Mechanical Signals Main/Subsidiary Signal, Upper Quadrant Type 

Mechanical signals were the first form of signalling to be introduced on the railway after hand signalling. The signal is a lever mounted on the signal post. This is moved by a wire, which is connected, to levers in the signal box

Electrical Signals - Multiple Aspect Main Type 

Electrical Signals are used to indicate to the train driver that the section of track ahead of the train is clear or occupied. This is achieved by multiple aspect lighting, which shows either a red, green or yellow aspect.

Junction Indicator Signals Position Light Type 

The junction Indicator (Position Light Type) system indicates to the train driver when the points are set to divert the train from the main route.

Automatic Level Crossing, Automatic Half Barrier Type 

Level crossings provide a means for road vehicles to cross the railway at grade. There are a number of types of level crossing, which give different levels of protection and automation of operation. This report deals with automatic crossings in general, and the automatic half barrier type in particular.

Manual Level Crossing, Manually Controlled Barrier with CCTV Type 

Level crossings provide a means for road vehicles to cross the railway at grade. There are a number of types of level crossing, which give different levels of protection and automation of operation. This report deals with manual crossings in general, and the manually controlled barrier with CCTV type in particular.

HVI Track Circuits 

Track circuits are electrically operated systems used to detect the presence of a train on a section of track.

Reed Track Circuits

Track circuits are electrically operated systems used to detect the presence of a train on a section of track.