Absolute Block
DAVID ALLEN looks at Absolute Block, a method of working in the UK since the 1840s, and which is set to remain in use for some years to come
DAVID ALLEN explores Absolute Block - a method of working in the UK since the 1840s, and set to remain for many years to come.
First advocated by the Board of Trade (BoT) in 1852, Absolute Block (AB) was a major improvement on the existing ‘time interval’ working. Controlling trains based on the time since the previous train passed was far from an exact science. The ‘All Clear’ given by the railway policeman (predecessor of the signaller) didn’t always mean ‘All Clear’. The previous train might be struggling to keep to schedule, or (worse still) have failed!
Although the benefits were clear, the largely advisory BoT had limited success introducing AB. There were complaints about the installation costs and concerns that it would slow movements down. But by 1889, when it was made compulsory on all passenger lines, it was in use on most routes.
However, AB wasn’t fool proof. Accidents in Clayton Tunnel (1861) and at Quintinshill (1915) make this clear. Both were the result of human error. Interestingly, as early as 1915, the famous Signalling Engineer Arthur Bound made a keynote speech to the Institute of Railway Signalling Engineers. He thought present methods (largely AB) were far from perfect, and that a better system based on track circuits could be used.
Despite this attack on AB, more than 100 years later it remains an important feature on the network. Clearly, vision without investment is not enough. AB remains in use because it has proved to be safe, reliable and adaptable. For example, before Line Clear can be sent, the Home signal must be at Danger and the appropriate Distant signal at Caution.
Similarly, all signal boxes (SBs) on passenger lines have what is called Line Clear Release (LCR). Although first used in 1870, it was only gradually introduced. The interlocking ensures the section signal cannot be cleared unless the block instrument points to Line Clear.
Following an accident at Welwyn in 1935, what is generally known as ‘Welwyn Control’ was developed. The signalman thought a train had passed through the section. As such, he sent ‘train out of section’ and accepted another train. Regrettably, the first train had failed in the section and the second train crashed into the rear. Welwyn Control is used to prove that the train has passed through the section, although observation of the tail lamp is required to prove it is complete!
Arthur Bound was a visionary. Up to this time, track circuits had played a very small role on the UK’s railways. Today, Track Circuit Block (TCB) working controls 72% of the lines on Network Rail. In addition, track circuits have been incorporated into more traditional methods to improve safety.
In many ways, TCB incorporates the same basic principles of AB. The BoT first advocated its use by stating: “No two trains shall be on the same line at the same time.” Today, the Rule Book gives the principle of AB working as “to prevent more than one train being in a section on the same line at the same time”.
With TCB working, the Section is now the distance between two stop signals. Entrance to the section doesn’t require the time-consuming communications associated with AB. Instead, track circuits are used to determine whether the route is available.
Over recent years, Axle Counters have also been used. Once again, the time-honoured principle of one train in the section is adhered to. A train is proved to have passed ‘complete’ when the number of axles leaving the section equals those entering.
The diagram (below) shows a typical double-track route divided into two AB sections, controlled by SBs A, B and C. To control the section between A and B, block instruments are found in both SBs. Likewise, the section between B and C is supervised by block instruments in B and C. B controls two sections, and needs two sets of instruments.
The indications on the instruments represents the position of train 2C19, approaching B on the Up Main and continuing towards C. The name (in this case ‘letter’) towards the base of each instrument is not that of the SB, but of the SB at the other end of the section.
Although all four Block Instruments illustrated can be seen on NR, no three adjacent SBs would display such variety. The BR Standard is the most universal. Dating from 1956, it is common on the former Eastern, London Midland and Scottish Regions. Its modular construction made it very adaptable. Although supposedly standard, what are known as ‘Left Hand’ or ‘Right Hand’ commutators can be found. Illustrated is the ‘Line Clear’ to the left. Both versions remain in use.
Clearly, standard doesn’t always mean standard. It seems BR didn’t want to confuse signalmen who had their Instruments replaced. A commutator was also made to facilitate Permissive working on freight lines. After the first train enters the section, the commutator is turned to ‘2’, ‘3’ or ‘4’… and so on. D irectly above the commutator is the ‘pegging indicator’. If the commutator is turned to ‘Line Clear’, the pegging indicator also points to ‘Line Clear’. In the case of A, the pegging indicator refers to the Down Main from B and the non-pegging indicator shows the condition of the Up Main to B. The BR Standard type is the only example that includes a unit for a bell and tapper. The other types have the bell and tapper mounted separately on the Block Shelf (not illustrated).
Even today, it is possible to find block instruments that only have indications for one line. It seems it wasn’t until the 1930s that the cash-strapped London & North Eastern Railway modified some to combine the indications for both directions in the same wooden casing. Note, this instrument displays ‘Line Blocked’ instead of ‘Normal’ - in signalling vocabulary, the same thing. Another difference is that the commutator is moved in a clockwise rotary way through the cycle. Less significant, but interesting, on the L&NER instrument not all the colours shown on the commutor match the indicators.
The Southern Railway Block Instrument
is confined to the former Southern Region of British Railways. The first examples date from a move towards standardisation shortly after the Grouping. Today, few are found.
AB on former Southern Region lines is limited. Although Sandwich SB works AB in both directions, the instruments are built into the panel. Maidstone West is the only one with two.
By far the most fascinating is the Glasgow & South Western Railway example, the design for which dates back nearly 150 years. After 1895, when the rules for block working were largely standardised, it became necessary to modify these previously two-position instruments to three-position. To enable them to show ‘Train in Section’, a disc was inserted alongside the signal arm. This is plain green, but changes to ‘On Line’ when a train occupies the section.
Fundamental to AB is the (Block) Section - normally the line between the Section Signal of one SB and the Home Signal of that in advance. Referring to the diagram, on the Up Main, Up Home C marks the end of the section from B. The Section signal controls the entrance to the next Section. Between the Home and Section signal is what is known as Station Limits. Station Limits is a historic term and might not feature a station - for example, Low Row (Tyne Valley) was closed in 1959 and Winning (Blyth and Tyne) has never had one.
Consider train 2C19 approaching on the Up Main. If the indicator for the Up Line in A is in the ‘Normal’ position and Train Out of Section (2-1) for the previous train has been received, A will send call attention (1) to B. B replies with 1 Beat.
A will send ‘is line clear’ for a Class 2 ordinary passenger train (3-1). Only if the line is clear and B doesn’t need to run another train first, B acknowledges by repeating (3-1). B now turns the commutator to ‘Line Clear’ and the needle applying to the Up Line moves to ‘Line Clear’. At the same time, the Up Line needle at A moves to ‘Line Clear’.
At this stage, A can clear the Up Main Section Signal for 2C19 to proceed towards B. When 2C19 enters the section, A sends train entering section (2). Once again, B responds by repetition, and turns the commutator to ‘Train on Line’. The Up Main needle at A will repeat this. Entering Section is one of the messages not preceded by call attention.
Assuming 2C19 is continuing towards C, B sends call attention (1) to C and follow a similar procedure to that outlined above.
Before B can send ‘train out of section’ to A, B needs to return the Distant signal to caution and the Home signal to danger. Assuming the section is not fully track circuited, the signaller must observe the tail light to confirm the train is complete and has not lost a carriage or wagon! In some locations, to allow train of section to be sent before the train passes the SB, tail light cameras are used. Similarly, the guard might be required to ring the signaller or press a plunger to confirm the fact.
A further form of protection is the Clearing Point. This is a location beyond the Home signal, and the distance varies depending on local conditions and whether the distant signal is colour light or semaphore. Once the train has passed the clearing point, ‘train out of section’ can be sent.
B now sends call attention (1) to A. After A responds, B sends train out of section (2-1) and turns the
Absolute Block remains in use because it has proved to be safe, reliable and adaptable.
commutator back to Normal. The respective needles in B and A will also point to Normal.
So far, the Train Register hasn’t been mentioned. Usually, Signallers working AB record the time that all bell signals are sent and received. Some SBs, such as those at Stockport, are ‘excused booking’. This does not mean a level of safety is missing. Here, the running lines are fully track circuited, and in this case the Train Register takes the form of an Incident Book.
On many secondary routes, AB remains in use. It is more difficult to build a business case to replace it, if the traffic can be handled and the equipment isn’t life-expired.
One thing is certain: compared with modern methods, AB is slow. As such, it’s not suited to heavily worked routes, although one obvious exception is the quadruple main line through Stockport.
AB is also labour-intensive. Consider the recent resignalling on the Hull line. Five SBs and four gate boxes were replaced by the Brough Workstation in York Rail Operating Centre (ROC).
As we enter 2019, AB continues in use on 11% of Network Rail’s route mileage. Looking at the past four years, on average 37 AB miles have been converted each year. If this rate of progress is maintained, it will take over 30 years to get rid of it!
In 2011, NR launched its Operating Strategy, which suggested that 452 SBs would be abolished by the end of 2018. Pleasing the dinosaurs among us, this plan was shelved, as over the years priorities and aspirations change and further projects compete for funding. Think of the money required to cover the rebuilding of the Dawlish sea wall!
Another scheme not planned in 2011 was electrification of the Blackpool line. According to the 2011 plan, theses SBs were due to last until 2029.
Some might also remember the Southern Region plan in 1969 to replace 555 SBs with 13 PSBs (Power Signal Boxes) - another bold initiative overtaken by events.
Although the 2011 Operating Strategy no longer applies, it is interesting to see how it would have affected the AB route mileage. There were 681 Block Posts controlling trains, ranging in size from small mechanical SBs to PSBs, Integrated Electronic Control Centres (IECCs) and ROCs. Gate boxes or shunt frames are not block posts.
In 2011, 413 SBs controlled AB in at least one direction. By the end of 2018, 117 had been abolished. This was a shortfall compared with the 220 originally scheduled, but considering present day restraints it remains a reasonable achievement.
Over the past four years, 148 route miles featuring 44 sections of AB have been converted to TCB.
In 2015, the East Sussex Coast Resignalling was finally commissioned. Five SBs and one gate box were closed, and five AB sections removed.
Later in the year, the North Lincolnshire Resignalling claimed 11 SBs and two gate boxes, and removed seven AB sections. This included the quadruple-line section between Wrawby Junction and Barnetby East. Between Barnetby and Brocklesby, of the three lines, only the Up Main was AB.
The East Notts Modular Resignalling started in 2015 and was carried out in two phases. In all, six SBs, two gate boxes and four AB sections were removed.
Early in 2018, the Preston to Blackpool Resignalling was commissioned. Five SBs were abolished, and three AB sections replaced by TCB.
The North Wales Coast Phase 1 Signalling Renewal was another major scheme in 2018. This claimed six SBs, one gate box and seven AB sections.
The Ferriby to Gilberdyke Resignalling was completed in November 2018, with six SBs, two gate boxes and five sections being consigned to history.
An intriguing survivor had been the short section between Banbury South and North. It was converted to TCB when both SBs were abolished in 2016. Another ‘island’ of AB was centred on Ashton Moss North Junction SB. Latterly, in the direction of Stalybridge, Ashton Moss SB worked AB to Manchester East SCC.
The recent Hebden Bridge to Bradford Resignalling rid West Yorkshire of its last two block sections. The relatively short section between Milner Royd Junction and Hebden Bridge was divided into two by Intermediate Block (IB) signals fitted by Railtrack in 1997.
Very much a unique loss earlier in the year was the section between Frodsham Junction and Halton Junction. After the Up line was removed in 1991, the Down line remained AB and only available for signalled movements between Frodsham Junction and Hatton.
NR has started its Cornwall Capacity Enabling Programme. Phase 1 West will include new IB signals in the existing AB sections - much-needed work when you consider that the section between Truro and Roskear Junction is approaching 13 miles.
The closure of Wennington Junction SB in 2006 created the longest section on NR. A stopping passenger train takes over 45 minutes to clear the 24 miles between Carnforth and Settle Junction. The section between Blair Atholl and Dalwhinnie is of similar length, but is divided up by IB signals. The shortest section separates Bedlington South and North. Remarkably, three of the shortest AB sections are at Stockport on the West Coast Main Line quadruple line.
Currently there are 1,127 route miles of AB. This figure includes sections which are AB in one direction. These include Hereford towards Moreton-on-Lugg, Hastings to Bo Peep Junction, and Aberdeen to Newtonhill.
Only NR’s Wessex Route doesn’t have any AB mileage. London North Western (LNW) has the largest amount with 359 miles, followed by London North Eastern and East Midlands (LNE, 284), Wales (177), Scotland (169), Western (78), South East (34) and Anglia (26).
The Anglia Route currently features 11 block sections, six of which are scheduled to be lost when the Lowestoft/ Yarmouth Lines Resignalling is completed in 2019. Remarkably, London still has two AB sections between Dudding Hill Junction and Acton Canal Wharf SBs.
The South East Route has only nine AB sections. One, between Newhaven Harbour and Newhaven Town, is scheduled for early replacement. The Medway Valley was resignalled with colour lights when Train Protection and Warning System (TPWS) was fitted, but three AB sections remain in use here. Bognor Regis is a curious survivor - although Barnham SB was replaced in 2008, AB was retained towards Bognor.
There are 13 AB sections on the Western Route, mainly in Cornwall or the Worcester area. Interestingly, the longest AB section on the Western dates from 2011, when the Cotswolds Line Enhancements were
On many secondary routes, AB remains in use. It is more difficult to build a business case to replace it, if the traffic can be handled and the equipment isn’t life-expired.
commissioned. The original intention was for West Midlands SC to control the resignalled and partly re-doubled line between Oxford and Worcester. However, all the SBs remained in use, and Moreton-in-Marsh continues to feature semaphores. The re-doubled 15-mile section between Moreton and Evesham was made AB, just as it was before it was singled in 1971.
The Scotland Route has 25 AB sections. The shortest is between Stirling Middle and Stirling North SBs, and is less than half a mile. Although most sections are found between Stirling and Aberdeen, the line between Annan and Kilmarnock is very interesting. Following electrification of the WCML to Glasgow, it was proposed to single much of the route. Defunct relay rooms dating for this can still be seen.
More recently, in 2005, the TCB section between Hurlford and Kilmarnock was converted back to AB and Hurlford SB fitted with a Block Switch, enabling it to be switched out during quiet times. When operated, the SBs on either side communicate directly.
The Wales Route features 30 AB sections, many of which are not in Wales! The Route is not a geographical entity, but an operational convenience.
Probably the most interesting of the Wales Route lines is that from Newport to Chester via Hereford and Shrewsbury, which crosses the border many times. Many of the SBs are among the oldest on NR.
The longest Wales Route section is between Gaerwen and Valley (14 miles). At one time there was a suggestion that Ty Croes gate box would be reinstated as a block post to break the section up. With Phase 2 of the North Wales Signalling Renewals postponed, AB continues in use west of Llandudno.
The LNE and LNW Routes are both wellendowed with AB - together they account for 643 miles of AB and 131 sections. One of the less well-known is between Glazebrook and Warrington Central on the line between Manchester and Liverpool.
AB working also remains in use at Dinting. This means that when a Manchester-bound train arrives at Broadbottom, the guard lets the Manchester East SCC signaller know the train has arrived ‘complete with tail light’.
The only AB section located at the southern end of the LNW Route is between Lichfield Trent Valley Junction SB and Alrewas, and it is shared with the LNE Route.
The last AB-worked triangular layout with a Signal Box at each corner is at Norton (near Stockton). On the Tyne Valley, there is an SB at Bardon Mill which is rarely used. When switched out, the section extends nine miles between Haydon Bridge and Haltwhistle.
In 1997, a new SB was commissioned at Worksop as part of Railtrack’s Project EROS. This required tail lamp cameras at Woodend Junction to allow Worksop SB to send ‘train out of section’ to Elmton & Creswell SB.
The average length of the surviving 219 block sections is 5.1 miles. The NR Routes with shorter than average sections are Anglia (2.4 miles), South East (3.8 miles) and LNE (4.2 miles). In contrast, the longest average sections are found in Scotland (6.8 miles), Western (6 miles), Wales ( 5.9 miles) and LNW ( 5.7 miles).