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| Track circuits and absolute block 03/11/2016 at 18:13 #87180 | |
TUT
532 posts |
From what I can gather, track circuits quickly proliferated even within absolute block areas - particularly within station limits. Indeed, it seems that most, or even all, absolute block areas that survived into Network Rail days were fitted with track circuits. What I'm interested in is any details on the history of the provision of track circuits in absolute block areas. What I'm most keen to understand is the rationale behind providing track circuits, but not converting the area to track circuit block operation. I'm not against it, I quite like absolute block, but I'm curious to know why it was that this method of working, which was devised before any method of train detection other than human eyes existed, and which seems primarily based on having signalmen observe the passage of trains and guard entry to the block section, survived having electrical train detection partially overlaid on top of it. What, in short, underlies the decision to install track circuits on the track under the control of (say for example) Wem signal box, leave the absolute block section to Harlescott Crossing un-track-circuited, and then install track circuits on the track under the control of Harlescott Crossing SB? Why the decision to persevere with block bells and instruments and all the rest of it for that bit of track? Also, as I say, I'd be interested in a timeline giving the history of installation. Log in to reply |
| Track circuits and absolute block 03/11/2016 at 19:27 #87185 | |
KymriskaDraken
963 posts |
Short answer: Money Longer answer: I don't know the areas in question, but maybe the decision was take to track circuit a section so that a signal box could be abolished without creating an overly long section. For example a signal box could be replaced with an Intermediate Block Signal. An IB signal needs to have a track circuit extending from the stop signal in rear of it up to the overlap of the IBS. This would be cheaper than converting the whole thing to TCB, especially if the train service didn't merit the conversion to TCB. Kev Log in to reply |
| Track circuits and absolute block 03/11/2016 at 19:37 #87186 | |
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Danny252
1461 posts |
Why decide to change? Adding track circuiting through several miles of block section costs a lot more than just doing station limits, and unless you're going so far as to invest in resignalling the line (more money and time!), you don't add much extra capacity and still pay the same staffing/maintenance costs. Track circuiting station limits does make more sense as it reduces some relatively large risks - with the potential for multiple trains in close proximity in various directions (only one at a time in a block section, hopefully) and the ability for the signalman to damage, derail, or collide trains through incorrect operation of equipment (not much chance of that on plain track with one train), the additional cost is deemed acceptable for the risk reduction. I can think of several recent incidents due to signallers incorrectly operating points, but not cases of having two meeting in a block section due to signaller's error. Log in to reply |
| Track circuits and absolute block 03/11/2016 at 19:41 #87187 | |
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TUT
532 posts |
Thank you for the responses so far :) So does anybody know which way round it was? I mean, did they install track circuits as an aid within station limits, and then develop track circuit block working on the back of that? Or was it a case of developing track circuit block working first and then realising that track circuits could also be helpful in improving safety in absolute block areas where full conversion to TCB just couldn't be justified? Log in to reply |
| Track circuits and absolute block 03/11/2016 at 20:34 #87189 | |
Steamer
3985 posts |
Often, the first piece of track to receive a TC would be a short distance approaching the home signal. This was designed to prevent a signalman forgetting a train stood at the home and giving another line clear.
"Don't stress/ relax/ let life roll off your backs./ Except for death and paying taxes/ everything in life.../ is only for now." (Avenue Q) Log in to reply |
| Track circuits and absolute block 03/11/2016 at 21:41 #87190 | |
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JamesN
1608 posts |
Track circuiting within station limits was generally to provide point locking; and as others have said to prove Train had passed through section (Welwyn controls), and as an aid to the signalman as to what was going on outside his window. Station areas were much more likely to have an electricity supply for the track circuits to use than 10 miles of remote railway between villages, so that consideration has to be factored as well.
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| Track circuits and absolute block 04/11/2016 at 09:31 #87191 | |
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Danny252
1461 posts |
I've been led to believe that early track circuiting technology wasn't up to handling sections several miles long, hence why it appeared in station areas first. The idea of train detection wasn't new, with fouling bars, depression bars, etc. existing before track circuiting.
Last edited: 04/11/2016 at 09:31 by Danny252 Reason: . Log in to reply |
| Track circuits and absolute block 04/11/2016 at 11:47 #87192 | |
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kbarber
1742 posts |
I think it's rather a complex story all in all. I would challenge both Steamer and James. So far as I can tell, the earliest installations were within station limits (that is, in advance of the home signal). It wasn't uncommon to find a crossover within the TC in a position that a movement stood at the protecting signal was on the TC concerned, so the points certainly weren't locked by the TC. (As the signals concerned were invariably shunt signals they wouldn't be track locked anyway.) On the Midland, at least, TC provision was considered an aid to the signalman at locations where a movement might be easily forgotton, whether due to visibility constraints (including the possibility of weather conditions making visibility difficult) or traffic conditions that might distract the signalman while he waited clearance. There were some locations where the TC(s) began well in advance of the box (and of most or all of the points worked therefrom) and extended to the starter(s), usually where the starters were a long way in advance of the box. On the MR, a programme of providing such TCs was started as a response to the Hawes Junction collision of 1910 http://www.railwaysarchive.co.uk/documents/BoT_Hawes1910.pdf Whether a 'berth' track was provided was, I suspect, a bit of a hit-or-miss affair and, again, I would think it depended on visibility. Where there was an 'interlocked'block system (such as Sykes Lock & Block or Rotary Block), a berth track would appear only in case of real need since the block system was considered to give all the protection necessary (and indeed signals were fitted with 'TC diamonds' to indicate there was no need to carry out Rule 55/Section K). Lots of boxes like that on the Midland, and a number of those layouts lasted until resignalling. Sometimes, track circuit numbers make it clear that TC provision took place in stages, perhaps station limits TCs to start with, then the realisation that a berth track would be useful, then splitting down a station limits track to allow locking of points by TC (not difficult to imagine 'one on the floor' provoking that kind of response), then additional TCs for IB signals, then a further round of provision in station limits... Development of TC Block is a whole different story. Many of the early power signalling installations retained AB working between boxes. The big exception was the LSWR Woking - Basingstoke scheme of 1904, using the BPRS low-pressure pneumatic system, with automatic home/distant combinations at regular intervals (effectively 3-aspect mas with semaphores). That, of course, was US technology and in its native land would have been used in conjunction with train control by a dispatcher instructing local tower operators by telegraph/phone. But I believe that, over here, it used train description by block bell only, without instruments (in other words, pretty close to TCB as we know it). That belief is based on nothing more than an interior photo of Brookwood, showing the BPRS slide frame and a series of block bell plungers (Southern style), but no block instruments. What I don't know is whether all the automatic sections were indicated in a signalbox (UK standard since the 1920s or so) or whether they were unindicated, with 'stop and proceed' working at auto signals (US standard, also in France and on London Underground and used on the Euston - Watford DC lines signalling of the 1930s). I have an idea the next such installation was Marylebone - Neasden South in 1923, this time with 3-aspect colour light signals. Between Marylebone Passenger and Marylebone Goods Junction boxes, the working was standard AB, with the auto sections north of Goods Junction. In 1967 the Marylebone boxes were replaced by an LMR standard (type 15) box of 60 levers. Working from that box to Neasden South was standard TCB with description by block bell; I have no reason to believe that was any different to the working before 1967. There was no 'stop and proceed' working from 1967, all signals were provided with SPTs and normal rules applied. I don't know what the situation was before 1967. I'm afraid I don't know much about the history of IB (Intermediate Block) signals. As Kev says, they allowed the abolition of a single intermediate box without reducing line capacity, but an IB in advance of the controlling box didn't require full track circuiting of the intervening lines - only from the starter of the controlling box to a point 400 yards in advance of the IB home. But that was only practicable where the box to be abolished was nothing more than a 'break-section' box provided to increase line capacity, which wasn't that common before the mid-1960s. (There were some areas, the London Tilbury & Southend on the London side of Barking, for instance, where IBs were provided in existing fairly short sections simply to increase line capacity; an accident report from 1959 http://www.railwaysarchive.co.uk/documents/MoT_EastHam1959.pdf suggests the signal spacing may have been even closer than the present setup you'll see on the LTS sim (though not as close as under the 1961 signalling).) Having an IB in rear of the controlling box did require full track circuiting so train out of section could be given before the train passed the box (such places had special authority to clear back without seeing tail lamp, but if the train subsequently passed without a tail lamp 'Obstruction Danger' was to be sent). Such instances were relatively rare and tended to reflect special local requirements. One such was Acton Wells, where Friars Place up IB was in rear of the box. The reason for that was that AW up home was close to the junction and I suspect Regulation 5 (warning arrangement) had previously been authorised from Friars Place, therefore AW had to have control of the signal there in order that a warning signal could be provided. (Incidentally it was a bit of a monster of a signal, with the home, equal-height splitting distants below, the warning arm on the main post below the bracket and a detonator placer worked by both the home and warning levers, all on wire pulls something like 1000 yards long!) Quite why TCB wasn't adopted has to be one of the mysteries of the universe, but S&T engineers tended to be rather conservative at the best of times. There were also one or two oddities, particularly in the 1930s. I've already mentioned the DC lines. The LMS also installed a peculiar hybrid somewhere between Skipton & Hellifield, not quite TCB but not fully AB either (I believe Hellifield South had some facility to refuse trains when they were described but I don't recall the details). The power frame at Kings Cross, too, although working TCB to Belle Isle, had to accept each train after receiving the description (again, I believe, by block bell); there were acceptance switches (most definitely not block instruments) in KX box for the purpose, worked separately for each train just like a block instrument but not requiring 'Train out of Section' to be sent. Perhaps the most important thing, as Kev alluded to, is that technology was expensive in the early years and labour was relatively cheap (extremely cheap before WWI), so it was rarely worth replacing an existing installation. By the time it became worthwhile, companies tended to be rather cash-strapped and that limited the progress that could be made. Hence the fascinating hotch-potch we ended up with and which has now largely disappeared. Log in to reply The following users said thank you: postal, AndyG, KymriskaDraken, Steamer, TUT, BarryM |
| Track circuits and absolute block 04/11/2016 at 14:47 #87193 | |
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Stephen Fulcher
2080 posts |
Danny252 in post 87191 said:I've been led to believe that early track circuiting technology wasn't up to handling sections several miles long, hence why it appeared in station areas first. The idea of train detection wasn't new, with fouling bars, depression bars, etc. existing before track circuiting.The maximum effective distance of a single track circuit today still isn't that high, hence we have cascaded track circuits in large numbers. Axle counters are more practical for longer distances. Log in to reply |
| Track circuits and absolute block 04/11/2016 at 15:20 #87194 | |
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KymriskaDraken
963 posts |
Stephen Fulcher in post 87193 said:Danny252 in post 87191 said:How far is it? Assuming ideal conditions of course.I've been led to believe that early track circuiting technology wasn't up to handling sections several miles long, hence why it appeared in station areas first. The idea of train detection wasn't new, with fouling bars, depression bars, etc. existing before track circuiting.The maximum effective distance of a single track circuit today still isn't that high, hence we have cascaded track circuits in large numbers. Axle counters are more practical for longer distances. Kev Log in to reply |
| Track circuits and absolute block 04/11/2016 at 16:56 #87195 | |
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Stephen Fulcher
2080 posts |
That's one for the designers but usually less than a mile.
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| Track circuits and absolute block 04/11/2016 at 17:36 #87197 | |
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AndyG
1842 posts |
Stephen Fulcher in post 87195 said:That's one for the designers but usually less than a mile.Sounds about right. Once at TRE I came across a combined TC split into about 7 individual TCs, I think most were less than 1 mile, typically 1 to 1.2km but only the combined TC shown on the display. I can only help one person a day. Today's not your day. Tomorrow doesn't look too good either. Log in to reply |
| Track circuits and absolute block 04/11/2016 at 17:59 #87199 | |
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Steamer
3985 posts |
kbarber in post 87192 said:The arrangements towards Hellifield were very peculiar, and contributed towards an accident there in 1955. See report for details: http://www.railwaysarchive.co.uk/documents/MoT_Hellifield1955.pdf "Don't stress/ relax/ let life roll off your backs./ Except for death and paying taxes/ everything in life.../ is only for now." (Avenue Q) Last edited: 04/11/2016 at 17:59 by Steamer Reason: . Log in to reply |
| Track circuits and absolute block 04/11/2016 at 19:19 #87201 | |
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GW43125
495 posts |
Danny252 in post 87186 said:Why decide to change? Adding track circuiting through several miles of block section costs a lot more than just doing station limits, and unless you're going so far as to invest in resignalling the line (more money and time!), you don't add much extra capacity and still pay the same staffing/maintenance costs.Nope, but a famous accident where the signalman forgot two light locos stood at his starter and belled straight through for the Scotch Express and, well... Log in to reply The following user said thank you: TUT |
| Track circuits and absolute block 04/11/2016 at 23:28 #87204 | |
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clive
2789 posts |
kbarber in post 87192 said:From memory of the report on the Seer Green collision in 1981, there were three intermediate boxes between Gerrards Cross and High Wycombe - Seer Green, Beaconsfield, and Whitehouse - which had all been abolished. So in each direction there were two automatics and then an IBS (each with a distant in rear). So even though three-quarters of the distance had been track circuited, it still wasn't seen as worth track circuiting the last quarter. Log in to reply |
| Track circuits and absolute block 05/11/2016 at 08:02 #87205 | |
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kbarber
1742 posts |
clive in post 87204 said:kbarber in post 87192 said:Indeed, but I didn't want to confuse the issue even further :-)From memory of the report on the Seer Green collision in 1981, there were three intermediate boxes between Gerrards Cross and High Wycombe - Seer Green, Beaconsfield, and Whitehouse - which had all been abolished. So in each direction there were two automatics and then an IBS (each with a distant in rear). So even though three-quarters of the distance had been track circuited, it still wasn't seen as worth track circuiting the last quarter. I believe that was a situation that came about over time, as boxes were abolished over a signficant period. At one time, Gerrards had just the one IB, Seer Green, which subsequently became an auto when (IIRC) Beaconsfield was abolished. Apparently you had to be extremely careful when switching out there, it was enormously easy to do so with a train still in the auto sections. Log in to reply |
| Track circuits and absolute block 06/11/2016 at 14:06 #87228 | |
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kbarber
1742 posts |
Steamer in post 87199 said:kbarber in post 87192 said:The arrangements towards Hellifield were very peculiar, and contributed towards an accident there in 1955. See report for details: http://www.railwaysarchive.co.uk/documents/MoT_Hellifield1955.pdf That's the one... thanks. Yes, very peculiar and extremely open to human error. Log in to reply |
| Track circuits and absolute block 07/11/2016 at 14:54 #87243 | |
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Zoe
252 posts |
kbarber in post 87192 said:Development of TC Block is a whole different story. Many of the early power signalling installations retained AB working between boxes. The GWR kept AB for their 1930s installations (the first MAS schemes was in the Southall area in 1953) but I recently discovered that the 1950s St Pancras area scheme kept AB between the boxes. Was it mainly the LNER and SR who were keen on early TCB installations. Log in to reply |
| Track circuits and absolute block 08/11/2016 at 12:31 #87250 | |
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kbarber
1742 posts |
Zoe in post 87243 said:kbarber in post 87192 said:Any idea where I can find that information Zoe? St Pancras is an area I have a particular interest in and would like to know more.Development of TC Block is a whole different story. Many of the early power signalling installations retained AB working between boxes. Having said that, there was definitely TCB between Dock Jc and St Pancras, using a train describer. Clearing points were protected by slots on each others' signals. It makes sense that in 1957 there was AB between Dock Junction and Islip Street. Islip St had control of the former St Pauls Road Passenger crossovers which fell well within the clearing point of Islip St's first home on the down. The position is less certain on the up - I don't have access to diagrams of like date, but only to Dock Jc as it was after Islip St was abolished - but I suspect again there were connections within Dock Jc's clearing point. There were certainly connections within the clearing points at Islip St, Kentish Town Jc and Carlton Road Jc, hence AB working (or slots) would have been required at those locations. The position changed after Islip St was abolished (in reality demolished by an errant ecs train) in 1966 and Kentish Town Junction in 1969; the working between Carlton Road and Dock Jc was then TCB with trains described by block bell. Incidentally the working between Islip St and Farringdon (LT) cabin was AB with block bell only and the lines track circuited throughout; after 1966 that was transferred to Engine Shed Jc and remained thus until the Moorgate lines were taken out of use in 1979 or '80 for conversion to 25kV and BR signalling. The working north of Carlton Road remained AB using rotary block until 1978, then AB from Engine Shed Jc (where a panel was installed to take over from Carlton Rd) until Finchley Road was abolished in 1981. Having thought about it a bit, I'm not aware of any LMS or early LMR TCB installations. Certainly when full track circuiting and 3/4-aspect colour lights were installed north of Camden in the 1950s, everything was done with IB signals. Kilburn No. 1 controlled IBs both in advance and in rear at Kensal Green. On the up fast, there was a crossover within the clearing point of Kilburn's first home so that makes a lot of sense; perhaps it just seemed logical to let Kilburn have the up slow IB too. The North London line, too, used IBs both in advance and in rear between Camden Road and Dalston Western Junction. Both boxes had both, so to the drivers it was effectively 3-aspect mas but the signalmen had to work all signals for all trains and AB instruments (BR 'penguins' by the time I saw them) as well; you really had to be on your toes to keep up with the work, especially when freight trains were following closely through the sections as well as the passenger services. I suspect that wasn't installed until the 1960s at least, though it was definitely in by the mid-1970s. Which is a long-winded way of saying I suspect you're right about the LMS (and LMR). Log in to reply The following users said thank you: john_s, TUT |
| Track circuits and absolute block 08/11/2016 at 13:01 #87251 | |
Ltbuckler
9 posts |
The maximum length of a track circuit depends on the type being used and vary greatly. the main three limiting factors are; - the declining value of ballast resistance - the increasing value of rail impedance - immunisation/electrification requirements, including electromagnetic compatibility with trains. For example: A Standard AC Immune BR867 DC Track circuit with a 9ohm relay has a maximum length of 1200m on concrete sleepers and 700m on timber However an audio frequency type track(TI21/ebi) on normal power in a DC traction area would be 1000m etc. Generally it is normally anywhere between 800m - 2000m maximum length for a TC Log in to reply The following user said thank you: GeoffM |
| Track circuits and absolute block 08/11/2016 at 13:33 #87252 | |
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Zoe
252 posts |
kbarber in post 87250 said:Any idea where I can find that information Zoe? St Pancras is an area I have a particular interest in and would like to know more. The 1960 Sectional Appendix shows AB on the main lines even between St Pancras and Dock Junction (Other lines had North London Junction between the boxes). In 1969 TCB is shown between Dock Junction and Carlton Road Junction but strangely not between St Pancras and Dock Junction. Could this be an error? Last edited: 08/11/2016 at 13:37 by Zoe Reason: Sectional Appendix error? Log in to reply |
| Track circuits and absolute block 08/11/2016 at 20:51 #87254 | |
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kbarber
1742 posts |
Zoe in post 87252 said:kbarber in post 87250 said:Thanks for that. Seems I need to make enquiries. But AB would be very peculiar, given the number of slots on each others' signals (which would be the usual method of preventing the approach of a train in TCB areas), which leads me to think this may be an error. Certainly it was TCB when I visited St Pancras in mid-1978 and I can't imagine it having been changed since 1969.Any idea where I can find that information Zoe? St Pancras is an area I have a particular interest in and would like to know more. Log in to reply |
| Track circuits and absolute block 08/11/2016 at 23:14 #87255 | |
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john_s
31 posts |
Following on from the comments above, there is no shortage of cases to show that full track circuiting did not necessarily lead to the abolition of block instruments. As another example, the western end of the Woodhead route (Newton to Ardwick) was almost fully track circuited, but retained block instruments until resignalling in 1980’s (there was a bit of TCB at the end, but not much). Likewise the Stockport area – which remains AB even today, because points within clearing points mean it is necessary to be able to refuse trains. The same reasoning would apply to the Woodhead route. Turning the original question around, there were also cases where track circuit ‘filling in’ between signal boxes did indeed lead to automatic signals. Here are just three isolated examples, which I happened to have to hand. They are all from LNER territory: Beeston Junction 1927: http://www.lymmobservatory.net/railways/sbdiagrams/lner_beeston_junction_q75x.jpg Laisterdyke East: The automatic signal on the Up Main at Ducketts Crossing was a GNR summersault; I don’t know the date of installation, but the diagram is 1953: http://www.lymmobservatory.net/railways/sbdiagrams/lde_1953_q50.jpg I don’t know the history of the Sleaford South – Helpringham section, but this diagram is from before 1957: http://www.lymmobservatory.net/railways/sbdiagrams/sleaford_south_q60.jpg In the Sleaford South and Laisterdyke East diagrams, notice the automatic semaphore signals shown as ‘normally off’. In all three cases, it was possible for the next box (Ardsley North, Stanningley or Helpringham) to switch out, and then absolute block working applied to the next-but-one box (Ardsley Station, Bramley, or Blotoft Siding). Log in to reply The following user said thank you: kbarber |
| Track circuits and absolute block 12/12/2016 at 13:11 #90851 | |
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vontrapp
210 posts |
Crook LC SB didn't have TCs. If you look at the photo, you can see a stop signal just ahead of the gates, that signal was to lock the trailing points ahead. Crew Sorting Sidings North SB doesn't have any TCs and the signalman uses hand-signals.  Post has attachments. Log in to view them. Log in to reply |
