In railway signalling, an interlocking is an arrangement of signal apparatus
that prevents conflicting movements through an arrangement of tracks such as
junctions or crossings. The signalling appliances and tracks are sometimes
collectively referred to as an interlocking plant. An interlocking is
designed so that it is impossible to display a signal to proceed unless the
route to be used is proven safe. In North America, the official 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 such as points and derails, and may include 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' 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the signal to proceed is withdrawn and sufficient time has passed to ensure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 route has had opportunity to come to a stop before
passing the signal. History
Railway interlocking is of British origin, where numerous patents were
granted. In June 1856, John Saxby received the first patent for
interlocking switches and signals. In 1868, Saxby was awarded a patent for
what is known today in North America as “preliminary latch locking”. Preliminary
latch locking became so successful that by 1873, 13,000 mechanical locking
levers were employed on the London and North Western Railway alone.
The first experiment with mechanical interlocking in the United States took
place in 1875 by J. M. Toucey and William Buchanan at Spuyten Duyvil
Junction in New York on the New York Central and Hudson River Railroad. At
the time, Toucey was General Superintendent and Buchanan was
Superintendent of Machinery on the NYC&HRR. Toucey and Buchanan formed the
Toucey and Buchanan Interlocking Switch and Signal Company in Harrisburgh,
Pennsylvania in 1878. The first important installations of their
mechanism were on the switches and signals of the Manhattan Elevated
Railroad Company and the New York Elevated Railroad Company in 1877-78.
Compared to Saxby's design, Toucey and Buchanans' interlocking mechanism was
more cumbersome and less sophisticated, so was not implemented very widely.
Union Switch and Signal bought their company in 1882.
As technology advanced that served to augment the muscle strength of human
beings the railway signaling industry looked to incorporate these new
technologies into interlockings to increase the speed of route setting, the
number of appliances controlled from a single point and to expand the distance
that those same appliances could be operated from the point of control. The
challenge facing the signal industry was achieving the same level of safety and
reliability that was inherent to purely mechanical systems. An experimental
hydro-pneumatic interlocking was installed at the Bound Brook, New Jersey
junction of the Philadelphia and Reading Railroad and the Lehigh Valley Railroad
in 1884. By 1891, there were 18 hydro-pneumatic plants, on six
railroads, operating a total of 482 levers. The installations worked, but
there were serious defects in the design, and little saving of labour was
achieved. The inventors of the hydro-pneumatic
system moved forward to an electro-pneumatic system in 1891 and
this system, best identified with the Union Switch & Signal Company, was first
installed on the Chicago and Northern Pacific Railroad at its drawbridge
across the Chicago River. By 1900, 54 electro-pneumatic interlocking plants,
controlling a total of 1,864 interlocking levers, were in use on 13
North American railroads and this type system would remain one of two viable
competing systems into the future, although it did have the disadvantage of
needing extra single-use equipment and requiring high maintenance.
Interlockings using electric motors for moving switches and signals became
viable in 1894, when Siemens in Austria installed the first such interlocking at
Prerov. Another interlocking of this type was installed in Berlin Westend in
1896. In North America, the first installation of an interlocking plant
using electric switch machines was at Eau Claire, Wisconsin on the Chicago,
St. Paul, Minneapolis and Omaha Railway in 1901, by General Railway Signal
Company. By 1913, this type system had been installed on 83 railroads in 35 US
States and Canadian Provinces, in 440 interlocking plants using 21,370 levers.
Interlocking types Interlockings can be categorized as
mechanical, electrical, or electronic/computer-based.
= Mechanical interlocking = In mechanical interlocking plants, a
locking bed is constructed, consisting of steel bars forming a grid. The 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 could not defeat the locking. The first mechanical interlocking was
installed in 1843 at Bricklayers' Arms Junction, England.
= Electro-mechanical interlocking = Power interlockings may also use
mechanical locking to ensure the proper sequencing of levers, but the levers are
considerably smaller as they themselves do not 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 in an
arrangement of relay logic 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 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.
Interlockings operated solely by electrical circuitry may be operated
locally or remotely with the large mechanical levers of previous systems
being replaced by buttons, switches or toggles on a panel or video interface.
Such an interlocking may also 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. GRS manufactured the first all-relay
interlocking system in 1929. It was installed in Lincoln, Nebraska on the
Chicago, Burlington and Quincy Railroad. Entrance-Exit Interlocking was the
original brand name of the first generation relay-based centralized
traffic control interlocking system introduced in 1936 by GRS. The advent of
all electric interlocking technology allowed for more automated route setting
procedures as opposed to having an operator line each part of the route
manually. The NX system allowed an operator looking at the diagram of a
complicated junction to simply push a button on the known entrance track and
another button on the desired exit track, and the logic circuitry handled
all the necessary actions of commanding the underlying relay interlocking to set
signals and throw switches in the proper sequence as required to provide valid
route through the interlocking plant. The first NX installation was in 1937 at
Brunswick on the Cheshire Lines, UK. The first US installation was on the New
York Central Railroad at Girard Junction, Ohio in 1937. Another NYCRR
installation was on the main line between Utica, New York and Rochester,
New York, and this was quickly followed up by three installations on the New
York City Transit System in 1948. Other NX style systems were implemented
by other railroad signal providers. For example Union Route was the brand name
of their vacuum tube-based NX overlay system supplied by Union Switch & Signal
Co., and introduced in 1951. NX type systems and their costly pre-solid state
control logic only tended to be installed in the busier or more
complicated terminal areas where it could increase capacity and reduce
staffing requirements. In a move that was popular in Europe, the signalling
for an entire area was condensed into a single large power signal box with a
control panel in the operator's area and the equivalent of a telephone exchange
in the floors below that combined the vital relay based interlocking logic and
non-vital control logic in one place. Such advanced schemes would also include
train describer and train tracking technologies. Away from complex
terminals unit lever control systems remained popular until the 1980s when
solid state interlocking and control systems began to replace the older relay
plants of all types. = Electronic interlocking =
Modern interlockings 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. In many implementations this
vital logic is stored as firmware or in ROM that cannot be easily altered to
both resist unsafe modification and meet regulatory safety testing requirements.
At this time there were also changes in the systems that controlled
interlockings. Whereas before technologies such as NX and Automatic
Route Setting required racks and racks of relays and other devices, solid state
software based systems could handle such functions with less cost and physical
footprint. Initially processor driven Unit Lever and NX panels could be set up
to command field equipment of either electronic or relay type; however as
display technology improved, these hard wired physical devices could be updated
with visual display units, which allowed changes in field equipment be
represented to the signaller without any hardware modifications.
Solid State Interlocking is the brand name of the first generation
microprocessor-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", of which VPI, MicroLok, Westlock and Westrace, now Siemens, and
Smartlock are examples. Defined forms of locking
Electric locking "The combination of one or more electric
locks or controlling circuits by means of which levers in an interlocking
machine, or switches or other devices operated in connection with signalling
and interlocking, are secured against operation under certain conditions."
Section locking "Electric locking effective while a
train occupies a given section of a route and adapted to prevent
manipulation of levers that would endanger the train while it is within
that section." Route locking
"Electric locking taking effect when a train passes a signal and adapted to
prevent manipulation of levers that would endanger the train while it is
within the limits of the route entered." Sectional route locking
"Route locking so arranged that a train, in clearing each section of the route,
releases the locking affecting that section."
Approach locking "Electric locking effective while a
train is approaching a signal that has been set for it to proceed and adapted
to prevent manipulation of levers or devices that would endanger that train."
Stick locking "Electric locking taking effect upon the
setting of a signal for a train to proceed, released by a passing train,
and adapted to prevent manipulation of levers that would endanger an
approaching train." Indication locking
"Electric locking adapted to prevent any manipulation of levers that would bring
about an unsafe condition in case a signal, switch, or other operated device
fails to make a movement corresponding with that of the operating lever; or
adapted directly to prevent the operation of one device in case another
device, to be operated first, fails to make the required movement."
Check locking or traffic locking "Electric locking that enforces
cooperation between the Operators at two adjacent plants in such a manner that
prevents opposing signals governing the same track from being set to proceed at
the same time. In addition, after a signal has been cleared and accepted by
a train, check locking prevents an opposing signal at the adjacent
interlocking plant from being cleared until the train has passed through that
plant." Complete and incomplete interlockings
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 do not identify an interlocking as one
or the other, and rule books do not 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
do not 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. References
External links Calvert, J.B. "Principles of
Interlocking." Interlocking
Kleinstadt.zip "Full free version of an interlocking plant based on German Relay
Principles"