Approach locking and Comprehensive approach locking

[PJW]

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.
I believe in India that there is often an outer distant and there are often no track circuits between this and the inner distant. Therefore there is a "dead approach". Where there are track circuits then "approach locking" is provided and it is this which we in the UK would call "Comprehensive Approach Locking"- the locking releases without a time delay if all the tracks are clear.
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".

So "Approach Locking" is the maintenance of the locking after a signal has been replaced to danger until such time as it is safe to release it. Having given a promise to the driver by clearing a signal, we must honour it until the driver has been given the opportunity to respond to a change in aspect.

Therefore we hold the locking until:
a) the train has entered the route, or
b) the train has taken so long that we believe it must have stopped prior to the signal.
Comprehensive approach locking adds a third possible release:
c) there was no train within aspect sequence range of the aspect reverson, so the locking can be released immediately.

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.

Hope that helps