Everything about Interlocking totally explained
In
railway signaling, an
interlocking is an arrangement of signal apparatus that prevents conflicting movements through an arrangement of tracks such as junctions or crossings. The signaling appliances and tracks are sometimes collectively referred to as an
interlocking plant. An interlocking is designed so that it's impossible to give
clear signals to trains unless the route to be used is proved to be safe.
A typical railroad definition of interlocking is "an arrangement of signals and signal appliances so interconnected that their movements must succeed each other in proper sequence."
Configuration and use
A minimal interlocking consists of signals, but usually includes additional appliances like switches (points in UK parlance),
derails, crossings at grade and movable bridges. Some of the fundamental principles of interlocking include:
- Signals may not be operated to permit conflicting train movements to take place at the same time.
- Switches and other appliances in the route must be properly 'set' (in position) before a signal may allow train movements to enter that route.
- Once a route is set and a train is given a signal to proceed over that route, all switches and other movable appliances in the route are locked in position until either
- the train passes out of the portion of the route affected, or
- the signal to proceed is withdrawn and sufficient time has passed to ensure that a train approaching that signal has had opportunity to come to a stop before passing the signal.
Interlocking types
Interlockings can be categorized as all-mechanical, electrical (
relay-based), and electronic/computer-based.
Mechanical interlocking
In mechanical or power interlocking plants, a
locking bed is constructed, consisting of steel bars forming a grid. The
control levers that operate
switches,
derails, signals or other appliances are connected to the bars running in one direction. The bars are constructed so that, if the function controlled by a given lever conflicts with that controlled by another lever, mechanical interference is set up in the
cross locking between the two bars, in turn preventing the conflicting lever movement from being made.
In purely mechanical plants, the levers operate the field devices, such as signals, directly via a mechanical rodding or wire connection. The levers are about shoulder height since they must supply a mechanical advantage for the operator. Cross locking of levers was effected such that the extra leverage couldn't defeat the locking (preliminary latch lock.)
Electro-mechanical interlocking
Power interlockings also use mechanical cross locking to ensure the proper sequencing of levers, but the levers are considerably smaller as they themselves don't directly control the field devices. If the lever is free to move based on the locking bed, contacts on the levers actuate the switches and signals which are operated electrically or electro-pneumatically. Before a control lever may be moved into a position which would release other levers, an indication must be received from the field element that it has actually moved into the position requested. The locking bed shown is for a GRS power interlocking machine.
Relay interlocking
Interlockings effected purely electrically consist of complex circuitry made up of
relays that ascertain the state or position of each signal appliance. As appliances are operated, their change of position opens some circuits that lock out other appliances that have would conflict with the new position. Similarly, other circuits are closed when the appliances they control become safe to operate. Equipment used for railroad signalling tends to be expensive because of its specialized nature and
fail-safe design.
Relay interlocking systems were often used in large and busy stations that have to handle high volumes of train movements. Since the
1980s, new interlockings have tended to be of the electronic variety.
Interlockings operated solely by electrical circuitry may be operated locally or remotely. Furthermore, such an interlocking may be designed to operate without a human operator. These arrangements are termed
automatic interlockings, and the approach of a train sets its own route automatically, provided no conflicting movements are in progress.
Electronic interlocking
Modern interlockings — those installed since the late
1980s — are generally
solid state, where the wired networks of relays are replaced by software logic running on special-purpose control hardware. The fact that the logic is implemented by software rather than hard-wired circuitry greatly facilitates the ability to make modifications when needed by reprogramming rather than rewiring.
By whatever means of technology are used, interlockings are designed to ensure that no operation can be performed unless all prerequisites have been satisfied.
"
Solid State Interlocking" (SSI) is the brand name of the first generation processor-based interlocking developed in the
1980s by British Rail, GEC-General Signal and
Westinghouse Signals Ltd in the UK. Second generation processor-based interlockings are known by the term "Computer Based Interlocking" (CBI), of which
MicroLok is one example.
Special interlocking functions
Approach locking
This is to prevent the change of route ahead of a signal once the operator has seen a proceed aspect at the signal or has seen an aspect at a previous signal that would indicate to him that the former signal is displaying a proceed aspect. Provision must, however, be made for such locking to be released provided a reasonable assurance can be given that any movement, the operator of which has sighted a proceed aspect, will in the event of the signal being replaced to danger, either have come to a stand at the signal or will have run past the signal onto the track circuits which lock the points or level crossing. Such provision may often take the form of a timer device, for example if the track circuit has remained occupied the train must have come to a stand.
Complete and incomplete interlockings (U.S. terminology)
Interlockings allow trains to cross from one track to another using a "turnout" and a series of switches. Railroad terminology defines the following types of Interlockings as either "complete" or "incomplete" depending on the movements available. Although timetables generally don't identify an interlocking as one or the other, and rule books don't define the terms, the below is generally agreed upon by system crews and rules officials.
Complete interlockings allow continuous movements from any track on one side of the interlocking to any track on the opposite side without the use of a reverse move within the limits of the interlocking. This is true even if there are differing numbers of tracks on opposing sides, or if the interlocking has multiple sides.
Incomplete interlockings don't allow such movements as described above. Movements in an incomplete interlocking may be limited and may even require reverse movements to achieve the desired route.
Further Information
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