Titwala - The place where Automatic changes to Absolute and Vice Versa ! 
Seen in the image is the Semi Automatic Starter Cum Gate Signal of Titwala S-17 (Up Line) with Equivalent Calling On and Shunting Signals accompanied by the Feather for diversion to the loop line . This is the first Automatic (Semi - Auto) Signal on North East Line (Up Direction) heading out from Kalyan Junction . Notice the Board saying 'AUTOMATIC BLOCK SYSTEM START' . The section ahead of Titwala till Kasara (under Mumbai Suburban Network) works under Absolute Block Signalling .
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1) Usually such kind of Signals are provided at places where there is a Level Crossing just ahead of a Signal and a point (directing to a Loop Line) just ahead of a Level Crossing .
2) When all the conditions for Lowering the Signal are available including the Gate Closed for the Road Traffic the 'A' L.E.D. light is lit up . It means the Signal is working in Automatic mode and is taking the Aspect from the Automatic Signal just ahead of it .
3) When working under Manual Mode , the 'A' L.E.D. is extinguished .
4) It also has a Calling On Signal which is extinguished as of now . A Starter requires many conditions for lowering itself. It can't be lowered if the track on which the train is intended to be received is occupied or the said Overlaps for a Signal are not free from Obstruction or are undergoing conflicting movements . Under such a condition it is possible to Clear the Calling On Signal . The Loco Pilot must proceed at a very low speed of 5 kmph and should be able to stop his train short of any obstruction . This improves the reliability on the Starter as well as increases Utility of Track Section .
5) SH-17 is the Shuting Signal . All might be knowing about this .
6) A Normal Gate Signal can take only two Aspects :--
1) Red - Gate is open for Road Traffic .
2) Green - Gate is closed for Road Traffic .
A Caution Aspect maybe included to signify a Permanent Speed Restriction ahead or in some other condition .
7) Mind it the usage of A and the other marker AG comes only when the Signal is in Stop Dead Aspect .
Case 1 - 'A' Marker Lit Up but Loco Pilot encounters Signal at Red .
In such a case the Loco Pilot will treat the Signal as an Automatic Signal with its Aspect as Stop Dead . He will stop before the Signal for one minute during the day and two minutes during night and will proceed cautiously till he encounters the next Signal or if there is any obstruction he should be able to stop his train short of any obstruction .
Case 2 - When 'AG' Marker is lit Up and Loco Pilot encounters the Signal at Red .
In this case the Loco Pilot will understand that the Signal is working as a Normal Gate Signal in Automatic/Absolute Territory . He assumes that the Gate ahead of the Signal is not closed ( probably not working ) and hence he will wait for one minute during day and two minutes during the night before the Signal and will honk and start moving afterwards . He will then stop the train before the Gate and ensure that Gate is closed . If Gateman is available he will ask him to close the Gate and if the Gateman is not available then one of the train staff will ensure closing/opening of the Gate after which L.P. will Proceed further .
Case 3 - When 'A' is extinguished and 'AG' marker is not lit and Signal is Red .
The Loco Pilot will consider this as a Manual Stop Signal and will wait for further instructions (either a Calling On Signal or a written Memo) .
8) In case train is to be received in Loop Line and if the Conditions are satisfied for lowering the Signal with Gate Closed it will indicate a Caution Aspect with the Feather Lit Up . If train is to be received in Loop Line and suppose the Gate isn't working or some condition is not available for lowering the Signal then the Loco Pilot will wait for a Written Instruction from Station Master .
P.S. - Incase Gate isn't working then Gate is manually Closed using a Chain or a Siding Boom .
Many Thanks to Khalid Sir for his contribution .
Location - Titwala Station .
Date - 10 th December , 2017 .