Minimum distance for absolute block

[The Black Hat]

Hello all, 

 

I am after some advice for the signalling on my layout. Having been looking at the possible track layout for the town scene that I am building the spur for a branch line is further away that originally envisaged. To get to it will involve bi-directional working on the up line from the junction north of the station to the branch.

 

The down line after the after the junction has no branch access so can have a signal protecting the junction for shunting moves. 

 

My question is - is there a limit on the minimum distance that a block section would be? Would they be needing to be long enough to take a train length ie freight - or could you have one smaller and then have it so that longer trains can not enter this section if they are to be stopped and then foul the preceding point and so occupy two sections. 

 

Also, would you have to have a signal at each end of the block as I am thinking you would - I am certain of this. So in that case you would have a signal prior to the branch and a one before the north junction on the up line if it were possible to insert a small block section. If not then the signals would be prior to the branch on the mainline and the whole area would be one block section. 

 

My period of operating is relatively recent, say about the last 15 years.  Thanks in advance for any forthcoming advice. 

[Phil Bullock]

There certainly are examples of blocks too short to accommodate a train … Worcester Tunnel Junction to Worcester Shrub Hill Junction immediately comes to mind. 
 

If you look at the panoramic view here 

 

Worcester Shrub Hill shed 

 

For trains approaching Shrub Hill from the north there isn’t much more than a couple of loco lengths between Tunnel Junctions starter (with a fixed distant) and Shrub Hill Junctions home signals. 
 

I don’t remember trains being held at Tunnel Junctions starter however so perhaps there was a rule about tunnel junction not accepting a train unless SH junction was also clear. 
 

Am seeing a load of Worcester railwaymen tomorrow night as am doing a film show for the Worcester Locomotive Society will add that to the list of questions for them!

[iands]

From some facts and figures in the notes of a Block Class I attended in 2018, the shortest AB section is 9 chains, or if you prefer, 198 yards. 

[njee20]

The platforms at London Bridge are split into multiple blocks - signal on platform 3 visible here halfway down the platform, with the next one at the end of the platform on the gantry. Again obviously markedly shorter than a train. Presumably a factor in the ETCS.

 

[micknich2003]

On the Down, Beverley Station to Cherry Tree, the block section was the thickness of the signal post.. See attached.

[iands]

8 minutes ago, njee20 said:

The platforms at London Bridge are split into multiple blocks - signal on platform 3 visible here halfway down the platform, with the next one at the end of the platform on the gantry. Again obviously markedly shorter than a train. Presumably a factor in the ETCS.

 

But they will be 'signal berths' on the same TCB panel, not AB as per OP. 

[Michael Hodgson]

27 minutes ago, The Black Hat said:

Hello all, 

 

I am after some advice for the signalling on my layout. Having been looking at the possible track layout for the town scene that I am building the spur for a branch line is further away that originally envisaged. To get to it will involve bi-directional working on the up line from the junction north of the station to the branch.

 

The down line after the after the junction has no branch access so can have a signal protecting the junction for shunting moves. 

 

My question is - is there a limit on the minimum distance that a block section would be? Would they be needing to be long enough to take a train length ie freight - or could you have one smaller and then have it so that longer trains can not enter this section if they are to be stopped and then foul the preceding point and so occupy two sections. 

 

 

 

I think you might get more helpful practical answers by posting an image of your track layout.

The simple if unhelpful answer to your question is indeed yes, there is a minimum length for an absolute block section and it's Zero!!

 

It might be more constructive to ask what signalling would be likely for your layout and era.  If it was historic and signalled under classic AB the bidirectional bit is highly unlikely.  Single line branches were typically signalled under staff & ticket (usually supplemented by AB), or token working, and "single lead juntions" were a no-no, as the old Board of Trade insisted on double track across the junction only then dropping to single track. 

 

But your era is last 15 years, and such junctions which are cheaper because they involve fewer point baldes and crossings became permitted and even fashionable in recent years (before falling out of favour again following accidents), and bi-directional working on double track is now commonplace, but typically signalled under track circuit block (TCB) with colour lights rather than semaphore and AB.

[The Stationmaster]

Agree with Mick above.  The minimum length of a block section could indeed be the thickness of a signal post (although if you want to be pedantivc it is ttheh thickness of a signal arm.  There were various examples around the country of such very short Absolute Block sections (and no doubt some very short Permissive Block Sections as well) around the country.

 

The next question - about a stop signal at each end of a block section is simple as the answer will be yes, even if the two stop signals are actually thesignal as illustrated in Mick's example posted above.  But is that what you meant or were you also, in effect, asking about two stop signals applying to the same line at a signal box (block post) because that is operationally important as you need two such signals to create Station Limits.  And certain things, such as propelling are permitted in Station Limits but are not normally permitted in a Block Section.

 

But as Michael Hodgson has already pointed out a track layout sketch, ideally with some dimensions added, is what we really need to come up with the answers you need.  However the biggie is in your final point where you say 'the past 15 years' which can take us into very different territory as we are well into the era of rationalisation and, on just about any former BR region it is highly unlikely that the track layout would be controlled by more than one signal box so we can forget block sections and think about Signal Sections (the section of line between two successive stop signals) instead - and there will probably then be a reflection of past Regional practice still surviving.  

 

So place is as important as era.

[Jeremy Cumberland]

As a general rule, railways aimed to have as few signal boxes as possible. The overriding limit (at least before point motors came along) was the distance between the signal box and the furthest away sets of points: 180 yards in early days rising to 350 yards (by the early 20th century, I think). The practical limit was less than this with heavy flat bottom rail, but by then point motors would have been added as part of a track upgrade. What distances are involved in your layout?

 

Having bi-directional running within station limits, with both the crossover north of the station and the branch junction being operated by the same box, would be no problem at all. Having bi-directional running between two signal boxes would be unusual, and if the distance were great enough to require two boxes, why wouldn't there be a crossover at the branch junction? If the track were simplified in recent years, then this is likely to have been part of a wider resignalling, which I expect would have closed the branch junction signal box (and quite likely the station signal box too).

[Wickham Green too]

4 hours ago, The Stationmaster said:

... 'the past 15 years' which can take us into very different territory as we are well into the era of rationalisation ...

Chances are the FORMER up line would have become the branch and the FORMER down line would have been singled and become bi-directional. ☹️

 

Edited November 6, 2023 by Wickham Green too
spilling

[Nearholmer]

3 hours ago, Jeremy Cumberland said:

Having bi-directional running within station limits, with both the crossover north of the station and the branch junction being operated by the same box, would be no problem at all.

That’s what I was going to suggest, provided that the distance doesn’t make in unfeasible.

[Stoke West]

46 minutes ago, Nearholmer said:

That’s what I was going to suggest, provided that the distance doesn’t make in unfeasible.

Craven Arms junction to Central Wales line facing crossover maximum mechanical worked distance from box then bi to junction controlled by ground frame operated by guard for facing moves (short term to closure which never happened) but sprung to run through with trailing moves . Later motor worked from box and since a new junction 2019 ? doing away with above .

Crediton double (now single) to two single lines

[Jeremy Cumberland]

1 hour ago, Stoke West said:

Craven Arms junction to Central Wales line facing crossover maximum mechanical worked distance from box then bi to junction controlled by ground frame operated by guard for facing moves (short term to closure which never happened) but sprung to run through with trailing moves . Later motor worked from box and since a new junction 2019 ? doing away with above .

Crediton double (now single) to two single lines

You mean Craven Arms Crossing (or Long Lane Crossing). Craven Arms Junction box was at the south end of Craven Arms station and closed in 1965.

 

There are a couple of photos of the Craven Arms Crossing signal box diagram before the latest resignalling on Twitter (as then was), which show the bi-directional line to be within station limits. The home signals must have been quite a pull at 1200+ yards (the up main distant was a colour light, and the branch distant was fixed). Signal 2, just beyond the far end of the crossover, is 355 yards from the box, which is, as you say, just about the limit for mechanically worked points.

 

[Stoke West]

13 minutes ago, Jeremy Cumberland said:

You mean Craven Arms Crossing (or Long Lane Crossing). Craven Arms Junction box was at the south end of Craven Arms station and closed in 1965.

 

There are a couple of photos of the Craven Arms Crossing signal box diagram before the latest resignalling on Twitter (as then was), which show the bi-directional line to be within station limits. The home signals must have been quite a pull at 1200+ yards (the up main distant was a colour light, and the branch distant was fixed). Signal 2, just beyond the far end of the crossover, is 355 yards from the box, which is, as you say, just about the limit for mechanically worked points.

 

After December 1965 , should have put a comma between Craven Arms and junction

[The Black Hat]

Hello all, 

 

Having read the discussion I am now sure that this will be a permissive working situation - all under the control of the one box on the layout at the other end of the station. 

 

To help explain ideas the photos requested are being shown below:

 

 

The station is off to the top right of the photograph. This shows the lines on approach. Going south (Up) is to the right here and into the station. The Up line has one line through the platform and a centre road through the station. The down line to the right just has one line through the platform and then comes round the corner. All lines here have access to each other given the track layout. Three signals shown top right all protect the junction and the start of this signalling section. 

 

To access the branch its proposed that the UP line, seen where the coach is, will be bi-directional. 

 

This photo shows the north station junction area before then going on around towards the town scene and off to the 'north'. 'Eleventh Earl of Mar' is shown facing towards the branch on what some might think is the wrong line - but it shouldn't be. The whole purpose of this track layout is to show a feel for the fact that the track has been rationalised. 

 

 

This photo shows the next board where the track will start to go past the town scene. The point shown here is purely for illustration purposes. It will have to be about half way down the next board but this then makes it possible for a section to be installed. I was thinking that it would have a signal for the junction point facing the station - and other on the up at the station junction near where the P2 is. But given the distance I am tempted to leave it as one larger section and given operators could be more on the other side of the layout it means theres less likely a chances of stock standing there and getting lifted.... 

Still, to complete the exercise it would be helpful to get advice on this given I now think its a signalled permissive working issue. 

 

 

 

 

[Jeremy Cumberland]

You appear to have 3-aspect colour lights at the station, so this would be Track Circuit Blcok (TCB) rather than Absolute Block, and the points and signals could be controlled from miles away.

 

In the up direction you will want signals before the branch junction, quite likely outside your scenic area. There will probably then be a signal about where the loco or red coach is, with some kind of route indication, but I couldn't tell you what kind. In the down direction, you just need a signal at the platform end with a junction indicator for the branch. If you can have wrong line movements on the station up roads, then these will need signals too, for exiting in the down direction.

[The Black Hat]

Would there be a need for a signal protecting the point for the branch heading off the layout? I am wondering as this could block the route off the layout depending on how big the next section would be off scene. Otherwise the train would be detected past the signal at the junction facing the station and occupying that section, so allow the section between the station and the branch junction to show as clear. 

[The Stationmaster]

Right so it is all Track Circuit Block with colour light signalling - which makes some difference to the context but is ideal for today.  What will make a difference is when the signalling was installed but we'll assume it is within the last 30 years or so and this means that the braking distance between successive signals will also play a part (a useful one for a railway modeller as it pushes signals much further apart meaning that some will be off scene).

 

So you will have Down direction signals at this end of the station approaching the complex of point work.  Which line will have Down direction signals is really governed by what you intend for the various movements you have in mind.  For example will there be a need for Down trains to depart from the Up platform (or from the centre road)?  Only you can answer that.  You appear to intend a train for the branch to depart from the Down platform which means that signal will need a Position 4 junction indicator (JI, aka 'feather to signal the move across to the Up line towards the branch junction.

 

Unless practice has changed you will need a signal for trains going towards the branch where the branch diverges from the Up line and it too will have a Position 4 JI to remind Drivers that they are diverging from the straight route.  Because of the distances involved this signal will need to be close to the toe of the points leading to the branch.  The branch is a single line of indeterminate length before the next crossing place or whatever so any movement from the station towards the branch will automatically prevent a movement arriving from it for however long the branch track layout dictates.  Providing this signal will make no difference to that.

 

In the Up direction you need only one signal thanks to the need for prototypical differences between most running signals.  Thus the two Up direction signals reading towards the branch divergence - and effectively right through to the station because they also protect the b-directional part of the Up line bi-directional line will be completely off scene,  

 

The only Up direction signal will be roughly opposite the  Down line end of the trailing crossover or may be even a bit further back than that.  It will have route indicators of some sort to read to whatever lines other than the Up platform line you want it to read to.

 

Colour light signal with Podition 4 Junction Indicator - note the method of mounting signals varied between BR regions and accirding to dighting requirements)

 

 

[Jeremy Cumberland]

3 hours ago, The Black Hat said:

Would there be a need for a signal protecting the point for the branch heading off the layout? I am wondering as this could block the route off the layout depending on how big the next section would be off scene. Otherwise the train would be detected past the signal at the junction facing the station and occupying that section, so allow the section between the station and the branch junction to show as clear. 

I am not quite sure what you are asking. For most movements, signals won't clear unless the entire route is unoccupied and no conflicting route has been signalled. For example, the off-scene up signal (main or branch) won't clear unless the route is clear right up to the station (an overlap distance beyond the station approach signal, the one Mike located opposite the Down line end of the trailing crossover). It might be possible to have a stationary train in the platform when a second train is allowed to approach, and then allow it past this signal with a subsidiary aspect (so that the second train can couple to the first, perhaps), but you'll need someone else to say exactly how this would be done.

 

In the down direction, trains going onto the branch won't be allowed out of the station unless the line is unoccupied all the way to the next passing place on the branch, and all the way to the off-scene signal on the up main. The off-scene up main signal will, of course, be held at danger (and the branch points will remain locked for the branch) until the down branch train has left the branch points track circuit.

[roythebus1]

Remember too that "permissive working" or permissive block only applies to non-passenger lines except  on the approach to platforms where a train can be signalled into an occupied platform using a subsidiary signal. The driver of a train entering such a section must be prepared to stop short of any train already in that section.

[roythebus1]

It would also be that the signal for the movement from the platform to the branch will display a yellow aspect with route indicator to remind the driver of the speed restriction and it will in effect act as a fixed distant for the next signal along the branch.

[The Black Hat]

Thanks guys for the information. I was thinking about placing two signals into the section for the line going to the branch but think I will be sticking with the original idea of just signalling before the branch point facing the station and junction - and then use the signals in the station. That way I can signal everything and still have things more or less right. It means a large block section for using the branch but this can add some movements onto the layout while mean that nothing it sitting in areas further away from where some operators will be. 

The knowledge and discussions have been really useful. I will be sure to include it in the Briganton layout thread.