Interlocking is a safety technique used in railway signaling to prevent conflicting movements by ensuring that signals and points are locked appropriately. Various devices such as detectors, stretcher bars, and slotting arrangements are used to enhance reliability and eliminate human error. Different control systems, including absolute block, automatic block, centralized traffic control, and automatic train control, are employed to manage train movements effectively and safely.

1. INTERLOCKING-DEFINITION
INTERLOCKING IS DEFINE AS THE TECHNIQUE ACHIEVED
THROUHG MECHANICAL OR ELECTRICAL DEVICES OR
AGENCIES BY WHICH IT CAN BE ENSURED THAT BEFORE A
SIGNAL IS TAKEN TO “OFF” POSITION FOR THE ROUTE AT
THE SAME TIME ALL OTHER SIGNALS AND POINTS (THE
OPERATION OF WHICH MAY LEAD TO CONFLICTING
MOVEMENTS) , ARE LOCKED AGAINST THE FEASIBILITY OF
SUCH CONFLICTING MOVEMENTS.

2. NECESSITY AND FUNCTION
• INTERLOCKING IS DONE TO PREVENT THE HUMAN ERROR
WHEN POINTS ARE SET AND SIGNALS ARE LOWERED OR
RAISED BY MEANS OF LEVERS.
• WITH AN INCREASE IN NUMBER OF POINTS AND SPEED OF
THE TRAINS, IT HAS BECOME ESSENTIAL TO ELIMINATE THE
ERROR ON THE PART OF HUMAN BEINGS AND TAKE ALL THE
PRECAUTION TO PREVENT THE POSSIBILITIES OF ACCIDENTS.
• IT IS ABSOLUTELY NECESSARY THAT ONLY THAT SIGNAL
SHOULD BE LOWERED FOR WHICH ROUTE IT IS MEANT.
• LEVERS ARE INTERCONNECTED OR INTERLOCKED, SUCH
THAT ONLY ONE POINT IN A PARTICULAR ROUTE CAN BE
OPENED AT ONE TIME AND WOULD GIVE SIGNAL FOR THE
SAME POINT.

3. DEVICE FOR INTERLOCKING
1) DETECTORS : IT IS ONE OF THE DEVICE TO ENSURE
A MECHANICAL RELATIONSHIP BETWEEN THE SETTING
OF POINTS AND TAKING “OFF” OF THE
CORRESPONDING SIGNAL.
THE DETECTOR IS SO ALSO CALLED BECAUSE IT AT
ONCE DETECTS ANY DEFECTS OR FAILURE IN THE
CONNECTION BETWEEN SWITCEHS AND THE LEVER
OR AN OBSTRUCTION BETWEEN STOCK AND TONGUE
RAIL.

4. OBSTRUCTION
THE SIGNAL REMAINS AT DANGER POSITION AND CANNOT BE TAKEN TO
“OFF” POSITION UNTIL THE DEFECT IS SET RIGHT.
DETECTION OF DEFECT

5. 2) STRETCHER BAR, LOCK PLUNGER AND LOCK BAR:
• THEY ARE USED FOR INTERLOCKING OF POINTS
• STRETCHER BAR HELPS IN INTERLOCKING DURING THE
TURNOUT.
• STRETCHER ARE INTRODUCE ON THE PERMANENT WAY AT
POINT WHERE TWO ROUTES MEET AND DUE TO STRESSES
CAUSED BY A TRAIN AT THE DIVERGENCE, THIS POINT IS MOST
VULNERABLE FROM POINT OF VIEW OF DERAILMENT.

8. 3) SLOTTING OF SIGNALS
• Slotting is an arrangement in which though the
lever operating a signal is in one cabin but the
actual taking “off” of the signal requires the
releasing of a control by another cabin.
• Hence a signal is to be operate and controlled
from two different cabin.
• Eg : switch interlock with fixed signal
(semaphore)
the arm will not move for proceeding sign until the switch is in
the right position.

9. 4) CONNECTING DEVICE AND TEMPERATURE
COMPENSATORS
• THE CONNECTING DEVICES BETWEEN THE
LEVER IN THE SIGNAL CABIN AND SWITCH
USUALY CONSIST OF 3.8 CM DIAMETER PIPE
OR SOLID ROD.
• THESE PIPES MOVE O A ROLLERS IN FRAME
FIXED IN CONCRETE AT 2M TO 3M INTERVALS.
• AT EVERY CHANGE OF DIRECTION, A SUITABLE
CRANK IS USED.

11. Method of control of Train movement
• One of the basic objective of signaling is to
control the movement of trains with a view to
ensure safety by prevention accidents.
• To achieve this goal, a particular system is
adopted keeping in view the three primary
aspects:
1. Safety of operation : system should be safe
2. Speed of operation : system should be Quick
3. Traffic density of operation : Satisfy traffic density

12. Various System for Control of movements of train
1) Absolute Block System
2) Automatic Block system
3) Centralize Traffic Control (C.T.C)System
4) Automatic Train Control (A.T.C) System

13. 1) Absolute Block System or Space Interval system or Lock &
Block system
• This system of working is most extensively used in INIDIA.
• In this System the entire track is divided into section called
Block Sections, separated by block station.
• All block Section are interlinked in series both
telegraphically and telephonically for verbal exchange of
information.
• Every block station has a Block Instrument that show as to
whether the section ahead is clear or reserved for a train
• A block station is under the charge of the station master
and a block section under the joint charge of two adjacent
block station

14. BLOCK SECTION BLOCK SECTION BLOCK SECTION BLOCK SECTION BLOCK SECTION
BLOCK STATION BLOCK STATION BLOCK STATION BLOCK STATION
Absolute Block System

15. Automatic Block System (Automatic Signal)
• This system is an improvement on absolute block system
and avoids the possibilities of accidents due to
negligence on the part of human beings.
• In this particular system, the signals are operated by the
trains themselves and therefore the trains can follow
each other between the two station.
• The section between the two stations is divided into a
number of blocks.
• When a train enters a particular block, the electric
current puts the signal at danger position (red light at
night) for the particular block until the train has gone
nearly two blocks ahead.

17. Advantage of Automatic Signal
• Due to automatic signaling, human error is
eliminated which lead to greater safety and
efficiency.
• Less no. of personnel for operating signals is
required which reduce the operating cost.
• Increased efficiency of operation will increase the
traffic density and reduce the requirement of no.
of locomotives and carriages.
(eg: A CARRIAGES SOME TIME IS ATTACEHD WITH TWO LOCOMOTIVE ENGINE)
• No block instrument is required (as in case of
absolute block system eg: token) which would
further reduce the cost.

18. Centralized Traffic control system
• Centralized traffic control means the
centralization of operations of all the points
and signals at the various stations on a section
of the railways at one single location and
concentrating the control over all the points
and signal indication into the hands of a single
official.
• This official thus has the control over all the
traffic movements on the section.

19. • This system increase the track capacity, so
encourages to run additional train. (for single line
operation)
• As the signal cabin are not required, there is great
saving in staff.
• Points and signals can be operated in few seconds
by means of thumb switches.
• The train can run at maximum speeds as the
driver has not to give signals, because the driver
is warned by means of a whistle or red light in his
cabin if approaching to wrong track or stop signal.
Advantage of C.T.C System

20. Automatic train control (ATC)system
• In this system not only the track is electrically
circuited, but there are electrical fittings at the
wheel brakes of locomotive.
• The advantage of these fitting is realized when
the driver fails to obey the danger position.
• The red light continues to lit and bell keeps on
ringing in the driver’s cabin for 5 seconds to warn
the driver. The break will gradually applied
automatically till the train comes to rest.

21. Working of A.T.C
• One part being installed to the track either
mechanically or electrically to apply the
brakes automatically to bring the train to stop.
• Other part is being applied in the cabin of the
driver to warn the driver such as light
continues to lit, siren or horn continues to
sound or ringing of bell