What is backlocking?

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!!

Is that a 57 lever fame?

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

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

*warms up keyboard*

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



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...

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 :

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....

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.

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

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.

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.

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!