Interlocking or Intra locking ?

Interlocking can be defined as "operation of a signaling element is decided by successful conditional achievement of another and thereby lock the the second until it is safe to release the lock "

Put it in simple words,to clear a signal to run a train in "Normal" or straight direction on the track ,Point Machine controlling the 'Normal' or 'Reverse' direction of the turn out track shall operate to Normal and detect to the desired position and ensure it stays at the Normal position(locked in that position ) until the train has cleared the track associated with the turnout and route locking releasing happened.These requirements are achieved by locking of one relay by another.

Infact there are 'interlocking' happening and achieving fail safe operation of a train .There are many generation of interlocking since its inception 1)Mechanical Interlocking ,2)Electro-Mechanical Interlocking ,3) Route Relay Interlocking 4) Entrance Exit (NX) Interlocking and Electronic Interlocking widely known as Solid State Interlocking or Computer Based Interlocking.

Interlocking system is 'One' of the brain depends on the type of system implemented .If block signaling is applied (Allowing only one train in a block and the train has to clear the block before another train can occupy the block) there is only "one vital brain " needed along with a 'non vital' control system or controller .The reason I used 'One' is that ,in modern technologies such as moving block system ,any number of trains can be in a section while maintaining a safe breaking distance between train. In order to achieve a moving block 'multiple' brain has to act together and intra locked to achieve this .This is not part of the scope for this article which will be described in Communication Based Train Control System (CBTC) with Automatic Train Control (ATC-Protection ) and Automatic Train Supervision Systems (ATS).Various Train control Systems (Entry -Exit Panel ) ,OCS ( One Control Switch ) and Automatic Train Supervision System will also covered with it functionalities in upcoming articles.

A control table(A Table describe the conditions to lock a signaling equipment)  are designed by a Signal Engineer based on the Signalling arrangement on the Track Layout ,widely known as Signalling Scheme Plan or Signalling Layout ,what ever it may be.

This table has multiple entries with conditions to clear a signal ,lock a point and other subsidiary equipments depends on the layout (Eg;To Clear a Signal ,required Point Machine Status ,Track Clear Status ,Conflicting train Movement status ,Level Crossing ,Fire Protection ,Flood and anything which required to be checked  as per the layout) .

A verified fail proof control table is the bible to create the " brain Interlocking" system .It can be for designing a mechanical interlocking ,relay based electrical interlocking ,or Ladder logic /Boolean logic for a computer based interlocking, Here I use the sequence of fixed block operation or Interlocking based on a Sydney Signal Box with few routes .

(https://www.transport.nsw.gov.au/industry/asset-standards-authority/find-a-standard/asa-standard-drawings-%E2%80%93-typical-signaling)

These logic can be achieved through relays or application software depends on the system. In modern days solid state interlocking (CBI) are getting common due to less maintenance cost on mechanical or electrical relay based system.

To clear four blocks with Signal 27,29 and 31 and 33 ,35 and 37 .Here 33(A) route lead to straight move with Signal 35 as destination(Point 501 Normal)  and 33(B) lead to Signal 37as destination  reverse move (Point 501 Reverse)

Routes with Signals

27--->29--->31--->33(A)-->35(Requires Point 501 Normal)

27--->29--->31--->33(B)--->37(Requires Point 501 Reverse)

Signal 27 ,29 and 31 belong to straight track and only one route possible in this track ,where as 33 signals has A route (Straight/Normal ) and B Route (Diverge /Reverse) controlled by a point leading to signal 35 and 37 respectively .I use '%' symbol to reflect relay drop status /(logic 0) and try to control with an Entry -Exit Control Panel .These relays interlock each other to achieve a safe operation by controlling each other and locking.

Control Panel Logic

MUR % (Machine in Use Relay ) is normally de energized(Drop)

33(R) Press button 33(entry of the route )

33(R)R Button Reverse Relay

33CeR Commence Relay (Note 33 is eNtrance)

33R(%) ------Let go 33 button (33 R button down)

33(R) R%--->Relay Dropped

MUR Machine in Use relay has" Stick" Function to stay until released

37 (R) Press 37 Button (Exit of the Route )

37 (R) R   Button Reverse Relay

37 FnR Finish relay

37 (R) R %  -- Relay Down

33CeR % - Relay Down

33FnR %- Relay Down

MUR % -Down

33CeR and 37 FnR calls B  route ----> 33(M) B U route leading to Signal 37 --->U stands for Route

If conflicting Routes Normal

If point correctly set or Free to set to desired position for B route ,that's Point Requires Reverse for B Route

33(M) B NLR %-Signal Route lock Relay will drop

33(M) B RUR -Signal Route Relay will stick

If Point Machine was lying in Normal to change to Reverse

501 NLR %   Point Normal Lock relay Down

501 RLR       Point Reverse Lock Relay Up

Note :- Look at Point Locking conditions at Control Table to create this logic

501 NWKR % Point Normal Detector Relay

501 IR  Point Isolating Relay Up ---IR relay is found in Sydney Interlocking Principles .

501 NWR %    Pont Normal Contactor

501 RWR     Point Reverse Contactor

501 IR      Isolating relay Drops

501 RWKR  Point Reverse Detection Relay Up

501 WZR % Point Free Relay Down

501 RWLKR Reverse Point Lock and Detection Up (Combined Logic of RLR and RWKR)

33(M) B UCR Signal Route Check Relay up

Note :- To Pick the Route Check Relay need to check the conditions required in the control table for the route to create the logic .

33 NGPR %   Signal Normal Repeater Relay .

33 NGPE % Signal Normal repeater Light indication for the Panel

33ALSR % Approach Lock Relay Down goes down

Note :- Approach Lock Relay is Normally Up Relay with three ladder logic path (One with Track Free status ,stick function and Time based Release ) .Will be defined in Control Table

33 (M) B USR (1 ) %   Route Stick No 1 Relay down

33 (M) B USR (2) % Route Stick No 2 Relay also stay down

Next Sequence is HR Relay (First less restrictive signal clear relay -Caution )

33 (M) B HR Caution Signal Relay Picks (Yellow) or Yellow Over Red for Sydney Sub Urban.

This relay control the first clear aspect ,of the signal for route clearance for the train to proceed with caution expecting a Normal Signal (Red at Next )

Each of these relays are controlling the status of other relays  and locking them until ready and safe to release !