Permissive signals

[programmes1]

Looking at the July quiz day 3 question which showed signal OB6 as a permissive is this correct as the starter is OB30, I thought that permissive signals had the same number as the starter followed by 00 etc can any of the tech people explain why this is not the case at Aldgate.

[Harsig]

programmes1 said:

Looking at the July quiz day 3 question which showed signal OB6 as a permissive is this correct as the starter is OB30, I thought that permissive signals had the same number as the starter followed by 00 etc can any of the tech people explain why this is not the case at Aldgate.

I should imagine it is to do with the points immediately in advance of signal OB30. Presumably they are too close to the signal to be allowed to move as the train approaches that signal even if OB 30 is at danger. Thus the last signal before OB30 (the pictured OB6) is provided with its own lever (No 6 unsurprisingly) and when reversed this lever mechanically locks 29 point lever in whatever position it happens to be in until the train is proved to be at a stand at signal OB30.
Were OB6 actually numbered OB300 then without a signal lever of its own there would be nothing to lock the point lever should an attempt be made to clear OB30 as the train approaches.
This is not to say that some alternative method of preventing the movement of those points could not have been used, it is just that this is the method selected in this case.

[JR 15secs]

Harsig is quite right and knowing where he works I'm not arguing OB6 works just as he said although re permissive signals having levers at Earls Court WB EC290 has a lever, perhaps the signal techs Tom or aetearlscourt can supply the answer.

[mrfs42]

I would suspect part of the reasoning that you can't 'swing' the overlap of OB30 after OB6 has been taken is due to having only the platform end of the points controlled (29) - the others in the westbound line are sprung, as well as the sheer physical closeness of the signals to the junction - if there was enough room I think that there would be the same layout as OB17, ROB15, OB15 heading out from platform 4 towards the Inner Rail .

The converging junction is protected by OB30 (platform) and OB31 (WB from Aldgate East) - I guess that further protection measures will/could be designed into the circuitry for OB6 and OB45 (Aldgate E WB starter) as over-run and flank protection.

Another factor for OB6 being OB6 rather than OB3000 could have been made at the interlocking design stage - OB6 with its own lever could 'release back' towards OB2 across the 9 - 13 points complex quicker than OB30 alone: allowing a following train sooner into one of the other platforms.

I'd guess that there has been some splitting of interlocking functions between OB6 and OB30 - some looking "ahead" towards A852^A/EJX852^B and some looking "behind" back towards OB2 or an early release on OB22. I suppose the answer lies in where the 'RB' or 'NA' contact bands lead.

I think that OB6 is a lot more complicated than just a draw-up, which I why I took a picture of it.

[Deleted]

I like this thread

Talking about overlaps and flank protection, how about the Inner Rail platform? I noticed that the starter was moved way back from the edge of the platform by about 1.5 car lengths - do similar issues apply here?

THIS THREAD is also useful - it talks about the removal of the converging crossover at Aldgate East Junction and the subsequent loss of flank protection at that site.

[mrfs42]

theonekea said:

Talking about overlaps and flank protection, how about the Inner Rail platform? I noticed that the starter was moved way back from the edge of the platform by about 1.5 car lengths - do similar issues apply here?

Partly: You've had a set of trap points removed from the IR platform at some point between 1946 and 1988. Whatever replaced those trap points has to replicate their functionality - that might include using such measures as timing sections, "delta-enforced releases (i.e. more stringent positional checking of trains, coupled with greater insistence on speed limits as opposed to just track circuit occupation) - in the other direction there will be a lot of differences also between (1946) OB28 [19points 'called' Normal] OB21 (20 points reverse) and the present situation with 9 points (platform 3 to IR) replacing [1946] 19 points.

In 1946 'route priming' and 'route calling' would not exist - but the bare bones of these principles would be present in the manually controlled interlocking. One of the great (dis)advantages of electromechanical interlocking is that converses have to be provided - but that allows more flexibility for the design engineer (you can put more conditions in for a lever to move from reverse to normal than vice versa).

The existence of OB6 rather than OB3000 allows better (quicker) 'route releasing'. If you understand why OB6 exists then you'll understand why the inner rail platform starter (OB17) is so far down the platform - the advantage here is that there is more space than on the outer rail platform - as ever signalling is a compromise between what the designers want and the characteristics of a train. It is not just a matter of not trusting the T/Op - you've got to design in a number of affordable and necessary redundancies.

With the case you're asking about it means that OB17 is a long way into the platform, speed is enforced from two signals back from the Inner Rail/Aldgate East points (Minories Junction) - enabling the approach speed to be limited and enforcing that speed by not having the platform starter (OB17) reading directly onto the inner rail (OB15 does)- this enables 'route priming' sooner from the two bay platforms and enhances the flank protection across Aldgate Junction - the control lengths and 'route calling' for OB14 ('junction home' on the inner rail - part of the flank protection for the signal you're asking about) use (I guess) a 15mph PSR from the platforms and the inner rail to platform 3 points being normally set to derail anything that SPADs from platforms 1 and 2.


Does that help? I've not long woken up from a snooze - so I'm guessing rather a lot and drawing on my memory of watching Aldgate Junction a fortnight ago.

[programmes1]

Have looked at the yellow peril OB6 is not a permissive (draw up) signal, that's what is in the YP (1/88) as there is mention of other signals in the area as draw up's. The signal looks like and operates just like a draw up the only mention is that with OB30 at danger OB6 will show a yellow aspect providing speed of train is reduced to 10mph or less (no mention of position of 29 points)the route is selected automatically once GF track circuit is occupied approaching platform 1, there was not even a route button provided in Farringdon cabin (what about Baker Street control room is there an icon?). The points which were removed were 19a/b which allowed a train from the OR/EB to go into platform 4.

[Harsig]

programmes1 said:

Have looked at the yellow peril OB6 is not a permissive (draw up) signal, that's what is in the YP (1/88) as there is mention of other signals in the area as draw up's. The signal looks like and operates just like a draw up the only mention is that with OB30 at danger OB6 will show a yellow aspect providing speed of train is reduced to 10mph or less (no mention of position of 29 points)the route is selected automatically once GF track circuit is occupied approaching platform 1, there was not even a route button provided in Farringdon cabin (what about Baker Street control room is there an icon?).

There are indeed route icons for OB6 at Baker St: 6(1) and 6(2).
6(1) clears OB6 with 29 points reversed and is not normally used as this would prevent any movements on the District through Minores Junction, while 6(2) clears OB6 with 29 points normal.

[mrfs42]

theonekea said:

I noticed that the starter was moved way back from the edge of the platform by about 1.5 car lengths - do similar issues apply here?

Yes. ;D

Now fully awake - the distance from the signal to the top of the platform ramp and the blockjoint does play a significant part.

Please apply Occam's razor to my previous answer.

[mrfs42]

harsig said:

(the pictured OB6) is provided with its own lever (No 6 unsurprisingly) and when reversed this lever mechanically locks 29 point lever in whatever position it happens to be in until the train is proved to be at a stand at signal OB30.

<just to check my thoughts> Is this tied in with DeltaXC (or more particularly a DeltaXC.JZR) - DeltaXC.JZR picked releases 29 lock both ways? DeltaXC being just before OB30 in the platform track?

.JZR being J (time) Z (special) - Delta XC picked and XC tracks down for <er..> 4.5/9 secs => .JZR picking?

[Tom]

relaydel said:

Harsig is quite right and knowing where he works I'm not arguing OB6 works just as he said although re permissive signals having levers at Earls Court WB EC290 has a lever, perhaps the signal techs Tom or aetearlscourt can supply the answer.

I've never looked into the reasons why. My guess is that it locks different things to the equivalent 'main' signal lever, in order to function as it should. My main involvement with 290's lever was to spend 45 mins in the evening peak watching to see if it moves and then if the (then still present) inductor worked!

Queen's Park also uses levers for draw-ups , and there it is related to locking different overlaps.

[mrfs42]

tom said:

Queen's Park also uses levers for draw-ups , and there it is related to locking different overlaps.

I think the same or similar situation pertains at Aldgate, though is conjoined with different degrees of protection (either flank or over-run): 28(N)KLPR or 28(R)KLPR.