Route Relay Interlocking / RRI

Route Relay Interlocking (popularly known as RRI), control the signals, points with the associated locking provided through sometimes complex wiring circuits using electromechanical relays as switches to control the end function e.g. signal displays, point movement and detection of the lie of the points. Thousands of relays are connected with any track movable device belonging to the area under the interlocking responsibility. These relays are either in the energised or de-energised states depending on the interlocking logic determining for instance points Normal or Reverse or a track section Clear or Occupied. The electro-mechanical status of the whole set of relays connected to the track identifies the “status of the railway” at any given time with constant communication between the trackside equipment and the signaller display system. The command to set and clear the route for the train is controlled via a signallers button control panel (usually for RRI an eNtrance eXit panel aka NX panel). When a command is given:

1. The RRI checks if the command is safe

AND

2. It activates the coherent set of relays, which lock the requested route through the electro-mechanical switching of the relay status.

The relays circuits are built using the Control Tables as the input document and uses the interlocking rules for the Logic. The Control table decides the possible movements of the train ensuring that every given route is fully protected from any potential conflicting routes and that it provides the correct information via the signals to the driver using that route. In a typical RRI installation the number of relays used to implement this type of logic is in the order of thousands of relays. That implies high costs and timescale for wiring installing, testing, commissioning and maintenance.

Solid State Interlocking Solid State Interlocking (SSI), Smartlock & Westlock

The Solid State Interlocking (SSI) system was first developed in the 1980s and jointly exploited by British Rail, GEC (now Alstom) and Westinghouse (later acquired by Invensys and now Siemens Rail Automation). Westlock is the property of Siemens Rail Automation & Smartlock was developed by Alstom. SSI is an interlocking system built using Electronics replacing traditional electro-mechanical relays. The same Interlocking rules or control equations used in RRI form the basis of this technology. The relays used to form the logic circuits in RRI are replaced by software variables and only the final Output driving relays are needed. A processor evaluates where the virtual relays (Software Variables) using the interlocking equations, which are now in digitized form either as Algorithms, Boolean equations or state charts in the processor memory. The control rules equations are modified as software algorithms and are stored in the embedded system memory. SSI offers many advantages over the oldest technologies, including:

• Minimal space taken.
• Entire logic circuits are simulated in software prior to commission.
• Relay wiring cost is saInstallation time comes down drastically.
• Maintenance is very easy.
• Verification and Validation of Software is a formal documented process.

Computer Based Interlocking – CBI

Computer Based Interlocking (also known as CBI) is the generic term for the most modern signalling interlocking products, implemented through computers rather than older technologies. This is essentially SSI but pertaining to the individual companies product which is derived from the original SSI format but has the advantages of increased processor speeds, increased capacity meaning much larger areas can be controlled via the interlocking and also improved user functionality. A modern CBI ensures higher availability, reliability and safety.

Mechanical Interlocking

Mechanical interlockings provide the control of a railway area through the movement of individual levers, physically connected via wires and rods to signals, points, level crossing gates or barriers.

Within areas controlled by mechanical interlockings, the railway is interspersed with several buildings (signal boxes) next to the track, containing manually or remotely controlled operated signal levers.

Even if this is an old solution, mechanical interlockings (both manual and remotely controlled) are still in operation along many parts of the UK main line railway network, particularly around rural areas and lightly used lines.