What is backlocking?

[paterson00]

I know that you TO guys would have been trained on this and where im working i doubt i will get a great answer as everyone is too busy to answer properly..

Anyone care to enlighten me?

[Colin]

No doubt I'll be pulled up if I'm wrong....

Where you have a set of points, you have three ways of locking them in position to prevent movement:

• Approach Lock - prevents the points moving as a train approaches
  
• Track Lock - prevents the points moving whilst a train is moving over the points
  
• Back lock - prevents the points moving until the train is clear, having passed over the points

In some lever cabins, when the back lock is active, you can only move the lever from the reverse position to the mid position - it will only go back to the normal position once the back lock has released.

If you want anything more technical than that, you'll have to wait for someone who really does know what they're talking about!!

[mrfs42]

This might help:

[Deleted]

Nov 25, 2009 9:59:58 GMT mrfs42 said:

This might help:

Is that a 57 lever fame?

[mrfs42]

11 - any number higher than that refers to autos.

[Deleted]

Nov 25, 2009 17:49:03 GMT mrfs42 said:

11 - any number higher than that refers to autos.

I was looking at 56B but I take it that was a TC for an auto.

Arrrrrrrr..............now I see an old typical!

[paterson00]

Nov 25, 2009 9:59:58 GMT mrfs42 said:

This might help:

Not without an explination Im afraid.. Im not there yet..

[Deleted]

Nov 26, 2009 4:27:01 GMT @paterson00 said:

Nov 25, 2009 9:59:58 GMT mrfs42 said:

This might help:

Not without an explination Im afraid.. Im not there yet..

*warms up keyboard*

I'll try, but most of my knowledge on this is based on reading the words of infinitely more learned individuals.

Nov 25, 2009 6:28:22 GMT Colin said:

• Approach Lock - prevents the points moving as a train approaches
  
• Track Lock - prevents the points moving whilst a train is moving over the points
  
• Back lock - prevents the points moving until the train is clear, having passed over the points

In the diagram shown by MRFS, approach locking would be active when a train approaching from the left entered track circuit A. When the train puts the track down after crossing the blockjoint, No.6 crossover would be locked into its current position and would be unable to move unless the signalman took a release on ZZ2. Note that while ZZ2 may or may not be on or off, because track circuits C through F are all part of 2's selection, pulling the lever from the frame would lock the crossover in its current position by locking No.6 lever in the frame itself; the signal doesn't have to be clear for the approach locking to work. Track locking would be in effect when the train puts track circuit D down, preventing No.6 crossover from moving at all. Backlocking would be in effect until the train has passed track circuit D and both C and D are up, preventing No.6 crossover from moving until the train has been proved clear of the crossover[1]. The same pattern (or not, depending on local rules and design) would apply in the opposite direction.

If the train is meant to shunt over No.6 crossover via ZZ9 up to A58, then of course the locking would be different, as the train would have to be proved to be at a complete stop[2] between ZZ9 and ZZ3. Once the train is proved to be at a stop, then the signalman can reverse No.6 crossover and pull 9's lever from the frame; when that happens the tracklocking will prevent No.6 crossover from moving again until after the train has cleared the crossover (track circuits D and G), and the backlocking will prevent No.6 crossover from being normalized and 9's lever from being restored in the frame until after track circuits D and G are clear. In this case however additional detection is needed because the backlock boundary extends from ZZ4 to ZZ9, as opposed to blockjoint G-H to blockjoint D-E (I have no idea how this works though). In this case approach locking appears to be replaced by proving the train has stopped. Just like in the first paragraph, the same pattern applies if the train is shunting from ZZ4 up to ZZ3.

[1] - Based on the diagram, it may be enough for track circuit D to come up. A more complicated site would have more restrictions.
[2] - There are a number of ways that this can be done, but I am not smart enough to guess which one would be appropriate.

fx: stampede of T/Ops and signalmen and infracos...

[railtechnician]

Generally trains have to clear points by a minimum of 40 feet from the tips of the switches. In order to do this a delta track circuit or position detector is placed a train's length + 40' beyond the points in the direction of travel. The delta or position detector has to pick up in order to release the backlock. No position detectors are shown on the diagram but of course the train's position would be known either by it passing ZZ3 and replacing its trainstop or by coming to rest wholly on E track and pulled up to ZZ3 for 15 seconds in order for ZZ2 to be released, unlocking 6W allowing them to be reversed for ZZ9 to clear.
By definition ZZ9 and ZZ4 must be at least 40' from the tips of the switches of 6BW and 6AW respectively. Moves from ZZ11 and ZZ4 would be done in the same way as for ZZ2 and ZZ9 with the train proved to have passed A58 or berthed on H track for a reverse shunt move.

[Deleted]

Nov 25, 2009 6:28:22 GMT Colin said:

In some lever cabins, when the back lock is active, you can only move the lever from the reverse position to the mid position - it will only go back to the normal position once the back lock has released.

Not sure about this Colin. Backlocks are usually about two thirds (or a bit more) of the way back to normal (referred to in S&T speak as the 'B' lock position). This is certainly the case on the big railway but it is some years since I worked directly in designing LT circuits. I will dig out some old circuits I have stored away (unless somebody beats me to it ).

EDIT: Thinking about this a bit more I think that with a remote air worked frame (Style N or V) that the lever air motor is 'puffed' from the reverse position just enough to break the reverse contacts (and thus break the signal selection circuit). The lever then waits for the backlock to pick and make its lock proving contact thus re-energising the air motor to put the lever fully back to normal. This is done to prevent the mechanical locking being strained. However, I still believe that the backlock is as I have described above (Awaits correction by a LT signalling engineer )

[mrfs42]

Nov 26, 2009 20:46:40 GMT @smeswdu said:

EDIT: Thinking about this a bit more I think that with a remote air worked frame (Style N or V) that the lever air motor is 'puffed' from the reverse position just enough to break the reverse contacts (and thus break the signal selection circuit). The lever then waits for the backlock to pick and make its lock proving contact thus re-energising the air motor to put the lever fully back to normal. This is done to prevent the mechanical locking being strained. However, I still believe that the backlock is as I have described above (Awaits correction by a LT signalling engineer )

Er.... 'KENT' Locks for the puff of air or am I thinking of something completely different? Lever backlocks are 'B' position still: 25° from N.

*scratches head based on TOK* I think:

Backlock release/protection against the backlock tracks bobbing would be by ∆s sited train length + 40' clear of any points (or a bit nearer if modern ∆ 'equivalents' are used) or 15 sec JRs 4½ sec LJRs (main/shunt) - LJRs being lock time delay relays that force a 15/4½ sec delay between a backlock track clearing and the lever 'B' position lock clearing or comparison between adjacent TCs being clear or occupied for the same length of times as above. These would be cold proved in the GR circuit for the preceding signal - ZZ1 off-camera to the left. Not to mention the usual trainstop TQ proof of it having gone back 'ON'.

[Colin]

Nov 26, 2009 20:46:40 GMT @smeswdu said:

Not sure about this Colin. Backlocks are usually about two thirds (or a bit more) of the way back to normal (referred to in S&T speak as the 'B' lock position).

I'm no signaller (though admittedly I have done some signaller training, but only the theory side of it as I wasn't deemed competent enough to actually work in a cabin!) - so I'm just a lowly driver trying to remember what I did pick up.

I'm sure the trainer said that if the lever wasn't in normal or reverse it could be considered to be in the mid position.

I may have remembered that wrongly, or the trainer may have been wrong, but that was the basis of my comment....

[Deleted]

on a v style frame the lever has many quandrants A through to E correct me if wrong im going back nearly 13 years now when the train has hit the replacing track of a signal it gets its first puff which takes it to mid or b position once then the train has picked up either a delta relay or sometimes when no points involved it has seen the trainstop come back to the fully on (up) position this picks up the GVCR (signal trainstop checking relay) and this picks up the electrical lock underneath the lever by moving a shaft up on the lock this picks up a LCC (a secondary device) this gives the lever its 2nd puff and allows the lever to go normal. On a manually operated frame it is the same put with no air so the lever would get stuck in the mid until the lock circuit has been completed thus picking the electric lock and signal op can now put the lever normal. There is sites which have a early backlock release facilty which allow the signal lever to go back abit early then normal to allow another route behind clear to allow the train say into annther platform or take another route completely

i.e the only one of top of head is the outer home at whitechapel on the e/b it was EN44/45 i think this had the facilty so when a train went towards whitechapel it could be normalised quicker to allow a train onto the ELL via St Marys Curve.

[Deleted]

I have now found a circuit for the backlock on a V style frame and the contact which gives the first 'puff' is an RE band. I suppose that the 'puff' is a bit unpredictable and the lever (shaft) could end up anywhere between the R and B positions but clear of the latter. The second 'puff' comes as I stated above, when the lever lock LCC contact is made.

Still searching for an equivalent N style frame circuit.

[Deleted]

Nov 27, 2009 21:04:20 GMT @aetearlscourt said:

i.e the only one of top of head is the outer home at whitechapel on the e/b it was EN44/45 i think this had the facilty so when a train went towards whitechapel it could be normalised quicker to allow a train onto the ELL via St Marys Curve.

There is a photo of this actually happening here:

www.wbsframe.mste.co.uk/public/Pictures/lt/LT_Whitechapel_Cabin_2.jpg

You can see the 'Delta' backlock light lit adjacent to the signal with the train having cleared the junction dead lock track circuit (you need to zoom in a bit on the photo )

[Deleted]

Nov 27, 2009 3:44:49 GMT Colin said:

Nov 26, 2009 20:46:40 GMT @smeswdu said:

Not sure about this Colin. Backlocks are usually about two thirds (or a bit more) of the way back to normal (referred to in S&T speak as the 'B' lock position).

I'm no signaller (though admittedly I have done some signaller training, but only the theory side of it as I wasn't deemed competent enough to actually work in a cabin!) - so I'm just a lowly driver trying to remember what I did pick up.

I'm sure the trainer said that if the lever wasn't in normal or reverse it could be considered to be in the mid position.

I may have remembered that wrongly, or the trainer may have been wrong, but that was the basis of my comment....

No offence intended in my comment (and I hope none taken). I would never wish to consider drivers as lowly. To a non -S&T person when a lever is neither Normal or Reverse then it could be rightly described as being mid-position as you describe so 10 out of 10 for your memory

[railtechnician]

Nov 27, 2009 21:04:20 GMT @aetearlscourt said:

on a v style frame the lever has many quandrants A through to E correct me if wrong im going back nearly 13 years now when the train has hit the replacing track of a signal it gets its first puff which takes it to mid or b position once then the train has picked up either a delta relay or sometimes when no points involved it has seen the trainstop come back to the fully on (up) position this picks up the GVCR (signal trainstop checking relay) and this picks up the electrical lock underneath the lever by moving a shaft up on the lock this picks up a LCC (a secondary device) this gives the lever its 2nd puff and allows the lever to go normal. On a manually operated frame it is the same put with no air so the lever would get stuck in the mid until the lock circuit has been completed thus picking the electric lock and signal op can now put the lever normal. There is sites which have a early backlock release facilty which allow the signal lever to go back abit early then normal to allow another route behind clear to allow the train say into annther platform or take another route completely

i.e the only one of top of head is the outer home at whitechapel on the e/b it was EN44/45 i think this had the facilty so when a train went towards whitechapel it could be normalised quicker to allow a train onto the ELL via St Marys Curve.

Early backlock release is known as permissive locking and Ealing Broadway has/had some of that on the route to/from District platforms to sidings as I recall.

[Tom]

Early backlock release and permissive locking are two completely different animals.

Early backlock release is just that - the backlock can be released early, either handing over the locking to another signal or route lever.

Permissive locking is where a signal lever mechanically locks another signal lever reverse, but with a small tolerance which enables the lever being locked to move just off the reverse position (but not as far as the 'E' position) in order to break any reverse bands and thus restore a signal to danger in an emergency.

[railtechnician]

Nov 27, 2009 21:35:17 GMT @smeswdu said:

I have now found a circuit for the backlock on a V style frame and the contact which gives the first 'puff' is an RE band. I suppose that the 'puff' is a bit unpredictable and the lever (shaft) could end up anywhere between the R and B positions but clear of the latter. The second 'puff' comes as I stated above, when the lever lock LCC contact is made.

Still searching for an equivalent N style frame circuit.

The quadrant as it is known from N to R is 60 degrees. There are five positions between N & R separated in degrees as indicated.

N-5-A-20-B-5-C-5-D-20-E-5-R

Hence there is little distinction between C (mid position), or B, the backlock position. On a 'V' style frame the lever is puffed to the B position from R but the ball catch bounces it back to the C position. It is not unknown for a lever to jam at the B position and lock itself out by preventing the backlock from picking up if the ball catch is worn out.
Release levers of course are locked at the D position and also sit in the mid awaiting the lock to pick up in the same way to go reverse. Point levers are locked at both A (Normal) and E (Reverse) positions.

A converted 'N' style power frame (N2) is arranged in exactly the same fashion.

[railtechnician]

Nov 29, 2009 14:25:57 GMT Tom said:

Early backlock release and permissive locking are two completely different animals.

Early backlock release is just that - the backlock can be released early, either handing over the locking to another signal or route lever.

Permissive locking is where a signal lever mechanically locks another signal lever reverse, but with a small tolerance which enables the lever being locked to move just off the reverse position (but not as far as the 'E' position) in order to break any reverse bands and thus restore a signal to danger in an emergency.

Tom,
I admit to being very rusty in my fifth year of retirement, it is true that if you don't use it you lose it. I thought there was more to permissive locking than you describe and that it was used for more than restoring signals in an emergency.

[Tom]

There may be, but that's the basic principle behind it. I think it may be used occasionally to simplify locking on individual levers, but the place I remember it from is Moorgate Met.

There's a shunt signal from the IR to OR, but the next signal a driver will see is the inner home signal. The shunt signal lever will mechanically lock the inner home lever reverse, but the facility is provided to bring the inner home's lever off reverse in an emergency.

[mrfs42]

Nov 29, 2009 15:06:35 GMT Tom said:

There may be, but that's the basic principle behind it. I think it may be used occasionally to simplify locking on individual levers, but the place I remember it from is Moorgate Met.

There's a shunt signal from the IR to OR, but the next signal a driver will see is the inner home signal. The shunt signal lever will mechanically lock the inner home lever reverse, but the facility is provided to bring the inner home's lever off reverse in an emergency.

Permissive Locking is a term only used (though I'm ready to be corrected on this) by the UndergrounD - it is a cognate for what other interlocking-type people would call Conditional locking. IIRC the conditional lock in this case is based on the points to the bays must be normal *and* both the trailing crossover IR to OR reverse and the inner home being reverse - conditionals are dependent of the position of a third lever.

Swinging Dogs eh?

[mrfs42]

However, back to the topic.....

The best (most succinct) description of backlocking I have seen comes from the 1930 supplement to the UERL 1928 Appendix:

Back Locking Circuits of Certain Signals

The back-locking circuits of a number of signals contain an additional relay. The operation from the Signalman's point of view is the same as in the case of other signals, except that in the case of the signal being returned to danger, after it has been pulled off, due to the track circuit bobbing, the signal lever will be back-locked if a train is standing on the approach track. The Signalman will, therefore, be prevented from pulling it off again and it will be necessary for the lever to be released by the Signal Lineman. If the signal fails to clear due to some defect it is possible to replace the lever.

However, that is perhaps not an entirely helpful paragraph; for brevity, the award must go to this paragraph from the route holding requirements - Backlocking The route must remain locked until the train has passed over all the facing and trainling junctions in the route.

In fact the operating requirements for route holding would be well repeated here:

• Route Testing It should be possible for a signalman to clear the route and replace it to normal as and when he wishes, providing that no train has approached the signal whilst it is displaying a proceed indication.
  
  
• Approach Locking The route miust be locked immediately a train passes the sighting point of a signal displaying a proceed aspect. The SP is a theoretical point at a calculated distance on the approach side of a signal. This distance is one that in which a train can be brought to rest at a danger signal and is usually the braking distance needed for the worst-braked train plus an allowance for Driver reaction time.
  
  
• Backlocking The route must remain locked until the train has passed clear of all the facing and trailing junctions in the route.
  
  
• Route not to be held if signal fails to Clear The signalman must have the facility of being able to replace the signal lever to normal if he has set up a route and the signal has failed to clear. This, even though a train is standing at the signal. The locking of the route in such a case would cause unnecessary delays and would serve no useful purpose. The signalman must be given the opportunity to try again to set up the route or to set up an alternative route.
  
  
• Signalman must be able to replace a signal to danger in an emergency The Signalman must be able in an emergency to replace a signal to danger even though he is not able to alter the route because it has been accepted by a train.
  
  
• "Fail Safe" The arrangements must be fool-proof. The route must not be released by a power failure, a 'bobbing' track circuit or by the improper manipulation of equipment.
  

HTH.

[railtechnician]

Nov 29, 2009 18:33:57 GMT mrfs42 said:

Nov 29, 2009 15:06:35 GMT Tom said:

There may be, but that's the basic principle behind it. I think it may be used occasionally to simplify locking on individual levers, but the place I remember it from is Moorgate Met.

There's a shunt signal from the IR to OR, but the next signal a driver will see is the inner home signal. The shunt signal lever will mechanically lock the inner home lever reverse, but the facility is provided to bring the inner home's lever off reverse in an emergency.

Permissive Locking is a term only used (though I'm ready to be corrected on this) by the UndergrounD - it is a cognate for what other interlocking-type people would call Conditional locking. IIRC the conditional lock in this case is based on the points to the bays must be normal *and* both the trailing crossover IR to OR reverse and the inner home being reverse - conditionals are dependent of the position of a third lever.

Swinging Dogs eh?

But conditional locking is called exactly that! The locking charts for frames will show for instance 3w7 as a condition lever 3 with lever 7 normal. My recollection of the permissive locking is that it looks as standard locking but is clearly marked 'Permissive'. The only permissive locking that I came across was on N frames and I don't recall many V frames without a swing dog or two. I am wondering if the term 'permissive' was more in keeping with the older frames.

In my days training as a lineman locking, like many other areas of signalling, was barely covered, the main emphasis being to turn out linemen for possessions and failures and many subjects being covered in later courses that didn't happen. Once passed out the lineman learnt from experience of dealing with failures and scrutinising whatever he needed to educate himself further. I did do a later lever frame course but that was more about replacing contacts and motors etc for those emergencies when a locking fitter was not available. Locking as such was the remit of the locking fitter.

[mrfs42]

Interesting - I wonder if it is merely a question of terminology - as my notes (which might well be wrong) usually say something like: Permissive Locking: reversing signal lever X is conditional on signal lever Y being reverse (a platform berth is available) or: reversing signal levers D or E or F is conditional on signal lever C being reverse (a siding berth is available) - and those notes are from a V frame

Somewhere there's also a note for Permissive Locking: reversing signal lever H is conditional on signal lever K being reverse (trains will not get stopped on the crossover).

[Tom]

Nov 30, 2009 10:44:27 GMT mrfs42 said:

In fact the operating requirements for route holding would be well repeated here:

W.H. Challis in the IRSE Green Book on 'Holding the Route'?

[Tom]

Nov 30, 2009 12:04:47 GMT mrfs42 said:

Interesting - I wonder if it is merely a question of terminology - as my notes (which might well be wrong) usually say something like: Permissive Locking: reversing signal lever X is conditional on signal lever Y being reverse (a platform berth is available) or: reversing signal levers D or E or F is conditional on signal lever C being reverse (a siding berth is available) - and those notes are from a V frame

Somewhere there's also a note for Permissive Locking: reversing signal lever H is conditional on signal lever K being reverse (trains will not get stopped on the crossover).

It sounds like the confusion arises from terminology. These days we teach that conditional locking is when a swing dog is used; lever 2 reverse locks lever 1 normal (no conditioning) vs. lever 2 reverse locks lever 1 normal only when lever 3 is reverse.

[mrfs42]

Nov 30, 2009 21:17:57 GMT Tom said:

Nov 30, 2009 10:44:27 GMT mrfs42 said:

In fact the operating requirements for route holding would be well repeated here:

W.H. Challis in the IRSE Green Book on 'Holding the Route'?

I should really have paraphrased it better - I was looking for my copy of the SR standards, but book 16 fell into my lap!

[mrfs42]

Nov 30, 2009 21:20:14 GMT Tom said:

It sounds like the confusion arises from terminology. These days we teach that conditional locking is when a swing dog is used; lever 2 reverse locks lever 1 normal (no conditioning) vs. lever 2 reverse locks lever 1 normal only when lever 3 is reverse.

Any mechanical locking without an escapement is 'dead locking' any mechanical locking with an escapement [1] can be conditional.

[1] Pedants: I'm not talking about 'reduced action' locking here - I'm thinking (citing Tom's example above) of when lever 3 reverse locks lever 1 normal when lever 2 is reverse.

Conditional locking introduces an element of 'choice' in the interlocking - normally mechanical locking is 'dead': there is no room for movement (escapement) lever 167 will always lock lever 212 normal (unpullable) whenever it (167) is reversed - however when or if lever 167 is pulled (reverse) and

(if anyone reading this is a bit confused) Conditional locking introduces a third lever (it can be more) into the chain of reactions: the net result is that in interlocking terms what would normally be a straightforward reciprocal relationship (if 1 is pulled you can't pull 2 nor if 2 is pulled you can't pull 1) is modified by another lever. It is best described with diagrams and mine are 300+ miles away!

However, rather than 'preaching to the converted' how about this as a possible suggestion for the origin of permissive locking - coupling up or consecutive movements in the same direction? Not quite the same as the same as conditional locking, but I can now think of a couple of ways where this *could* be achieved with 'dead' locking - the 'conditions' are fulfilled by the track occupancy and lever positions [2]; whatever its origins it is a curious bit of phrasing!

[2] as in electrically achieving the locking; unfortunately both examples I can think of for this are at Baker St, and prevent moves being able to depart in opposite directions from the same piece of track; which is almost *exactly* the opposite of permissive locking.

[railtechnician]

Nov 30, 2009 21:20:14 GMT Tom said:

Nov 30, 2009 12:04:47 GMT mrfs42 said:

Interesting - I wonder if it is merely a question of terminology - as my notes (which might well be wrong) usually say something like: Permissive Locking: reversing signal lever X is conditional on signal lever Y being reverse (a platform berth is available) or: reversing signal levers D or E or F is conditional on signal lever C being reverse (a siding berth is available) - and those notes are from a V frame

Somewhere there's also a note for Permissive Locking: reversing signal lever H is conditional on signal lever K being reverse (trains will not get stopped on the crossover).

It sounds like the confusion arises from terminology. These days we teach that conditional locking is when a swing dog is used; lever 2 reverse locks lever 1 normal (no conditioning) vs. lever 2 reverse locks lever 1 normal only when lever 3 is reverse.

Just to cloud the issue in terms of terminology, the BR signalling green book says little about mechanical locking but there is no mention of 'permissive' locking at all. It mentions dead locking, conditional locking and counter-conditional locking !

My understanding has always been that 'V' style frames are deadlock frames and while I am familiar with conditional locking I have never come across counter-conditional locking, at least none that is so called at LUL.