Approach locking and Comprehensive approach locking

[khalid]

Hi Mr.PJW,

Could please explain me about the difference between Approach locking and Comprehensive approach locking?
and where we can go for comprehensive approach locking?

I am no expert on Indian signalling but I have learnt enough to realise that we use words slightly differently and this is perhaps what causes confusion......
In the track circuited area, if any signal is replaced to danger the interlocking is maintained until it is safe to release it and I understand that this is called "back locking", although in the UK we'd call Approach Locking.
In fact the terms "back locking" and "Approach Locking" are both used and mean the same thing, although it is usual only to call "back locking" when the signal is controlled from a mechanical lever which is physically prevented (by an electric lock) from being placed fully Normal and thus still holding the mechanical locking of other levers whilst being sufficiently back in the frame that the signal has been replaced to danger.
On a panel or VDU, only the very old fashioned would refer to "back locking"; effectively the same functionality is called "approach locking".

On the Indian Railways,we use the terms Approach Locking and Back-locking in different contexts though they may be accomplished by the same apparatus in the mechanical lever installations.
Where mechanical lever interlocking is provided along with electrical locking,back locking refers to holding the route over which the train will travel till the train actually travels over it or the route is released after the release of the approach locking in case the signal has been put back to danger.
Hence,in normal situations,when the train passes the signal,the points ahead of the signal still need to be "held" (also called route holding or route locking in electrical relay interlocking). When the train passes the signal,it will be possible to normalise the signal lever but it will not go back completely and will stop about 3/4ths of the way.It will normalise completely only after the train has also passed over the points in the route and this is called releasing the back-locking on the lever.This is similar to Sectional Route Release or TORR(Train Operated Route Release)in a relay interlocking system.In fact in India,sometimes the term "back-locking" is used even in relay interlocking in the same context of the route being release by the passage of the train.
As I mentioned above,an electric lever lock is provided to prevent the lever from being normalised completely and this lock also functions while approach locking is active on the signal.
Thus,the terms Approach Locking and Back Locking cannot be used interchangeably.

Comprehensive A/L costs money to design, install and test so we only provide it where it gives good value. Hence where a signaller may need to reroute trains / change priority at short notice, so in and around the approach to major stations and critical junctions. Provision of Auto Working Facility can mean that a signal can reclear for a train that the signaller didn't intend, so signals with Auto buttons are often contenders for provision of comprehensive A/L.

Wherever Automatic signalling(Track circuit block)is provided,comprehensive approach locking is available but in case of Absolute Block system,the Home will always be under dead approach locking.

Khalid.