We often hear that “trains are delayed because of a signalling failure”. But why do we hear about it so much, and is it because there are more signalling failures?
Signalling is fundamental to the safe operation of the railway, ensuring that trains are spaced safely apart and conflicting movements are avoided. Railway signals are ‘traffic light’ devices, which tell a train driver if it’s safe to proceed along the track. As with road traffic lights, a driver shouldn’t pass a red signal.
Signalling failure refers to various things that go wrong, causing a train to be held up at a red signal. A stationary train quickly creates knock-on delays and, with the rail network busier than ever, the knock-on delay can be significant.
The total signalling asset count for the GB network is some 500,000 maintainable assets, situated in a wide range of environmental conditions which may affect their reliability and ability to be easily repaired.
Signalling equipment consists of a number of subsystems:
- Control systems – allow a signaller to set routes, using a mechanical lever frame, signal panel or screen-based system;
- Interlockings – ensure conflicting routes cannot be set; Points – route trains through a track layout;
- Signals – pass information to the driver – for modern digital systems, such as ETCS (European
- Train Control System) and the older RETB (radio electronic token block), this is provided via an in-cab display;
- Train detection – provide train positional information;
- Level crossings – allow footpath and road users to safely cross the railway.
Tracks are divided into ‘sections’ of track and normally only one train should be in a particular section at any one time, with signals positioned at the beginning of each section. The signals are controlled via the interlocking which ensures conflicting routes cannot be set by the signaller. An interlocking can be considered to be similar to a locked door into an unsafe building. Only when it’s safe to proceed will the door be unlocked.
However, the interlocking does not check that everything is safe for the passage of a train, it only checks from a signalling perspective. A section of railway track must be safe in other ways. For example, the distance between the rails must be correct and the track-bed must be capable of supporting the weight of the train.
Other systems, known as track circuits or axle counters, detect if a train is present in the section. To switch a train between tracks, signalling point machines move points, but only when allowed by the interlocking.
In a modern signalling installation, all these systems require some sort of electrical power supply to function and a large signalling area will require telecoms links to connect the subsystems together. Telecoms GSM-R ( mobile phones for railway control) and RETB base stations are also required to provide a communication path to the train, for movement authorities for ETCS and electronic tokens for RETB. Sometimes, these systems break and the signal turns red, even when there is no train in the section, or, for ETCS and RETB, no movement authority can be sent to the train.
Problems with track circuits and axle counters are common causes of signalling failures. A track circuit is a small current running between the tracks and trains, and an axle counter (as the name suggests) counts the axles and track wheels going in and out of a section – if the numbers match, the section is clear for the next train. Track circuits are susceptible to corrosion and rail contamination resulting in trains ‘disappearing’.
This is known as a wrong-side failure – a failure where the asset fails to an unsafe condition and one which must be thoroughly investigated so that the root cause is understood and addressed. This can cause further delay until the failure has been independently investigated and signed back into service by a competent person.
On the other hand, if flooding was to cause the track circuit to operate and turn the protecting signal to red with no train present, that would be a right-side failure – one in which the system failed to a safe condition.
Axle counters don’t suffer from rail contamination, poor ballast conditions or flooding conditions that can affect track circuits, but some axle counter systems do not respond well to heat and, in the event of a power supply failure, axle counters will come ‘back on’ and not detect if a train is present in the section. This requires a manual reset, which takes time and further delay.
A reliable source of electrical power is vital for modern signalling systems and uninterruptable power supplies (UPS), which take over when the main power supply is cut, are vital. A lot of investment is being made and, most recently, UPSs have been introduced on the West Coast. They are also provided for all new resignalling schemes.
In areas that don’t have a UPS, Network Rail is making the power supplies more reliable by replacing ageing cable. The health of the power supply system and cabling can also be monitored via remote condition monitoring equipment, supplemented by annual inspections. The objective is to fix problems before they become service-affecting faults.
A fail-safe system
Signalling systems are designed so that, if something stops working, such as a signal or a set of points, trains will stop before they reach that location. So, if there’s a power failure, the signal goes black and the driver knows not to pass a signal unless it has a green or yellow light. If a set of points should fail, the last signal before it will automatically turn red so no trains can pass. On some of the busiest lines on the GB mainline network, over 100 trains will pass over just one set of points every day, so they must be made as reliable as possible.
Like signals, points can fail. They might get clogged with debris or ice, the drive mechanism might fail or, in hot weather, they might expand too much. Unlike other systems, the point machine is a single point of failure with no back up or duplicate system. When points do fail, the system goes into ‘fail safe’ mode and the protecting signal is made red.
As points are now monitored remotely, in many cases, problems can be fixed before the points fail totally. Electric heaters and NASA-grade insulation have been provided to stop ice forming and jamming the mechanism, and protective covers have been fitted to 4,000 points and 2,500 point-motors to keep snow out and prevent damage from ice that can fall from passing trains. Improved training for installers and maintainers has also had a positive impact on points reliability.
Rails are painted white at critical points so they absorb less heat, which reduces expansion. Typically, a rail painted white is 5°C to 10°C cooler than one left unpainted. If points do fail, they can often be secured in one position so trains can pass over. This keeps lines open and trains moving and reducing delay. This will mean staff getting to site, which can be a problem in areas with congested roads or remote locations.
Cable theft
The theft of metal can be a big problem for the railway as thieves target signalling, telecoms and power cables, and even metal fences, to sell for scrap. Cable theft causes signalling failures and costs Network Rail millions of pounds each year. The total, taking into account the impact of freight delays and the cost to passengers who are late for work or have their day ruined, is even higher.
A lot of work has gone into tackling cable theft, including funding additional and undercover British Transport Police (BTP) officers, using CCTV to alert when people are on the network, installing new ways of securing cables, using forensic marking agents to make stolen cables easier to identify, and setting up a dedicated security team.
Network Rail, along with other industries, successfully lobbied the Government to introduce the Scrap Metal Dealers Act 2013. This requires scrap metal dealers to be licensed and gives local authorities the power to refuse unsuitable applicants and revoke licences. Police also have the power to close unlicensed scrap yards and sellers of metal must show verifiable identification which dealers must record and retain. In addition, cash trades for scrap metal are illegal and subject to unlimited fines.
All this has helped to make the sale of scrap metal accountable and to ensure all people trading scrap are doing so legitimately. The volume of cable theft has reduced, but it still takes place with some stolen cable now being taken overseas in containers for sale where the restrictions are less onerous.
Reduction in failures
Given the media and social media reports on the subject, it may be assumed that there is an increase in the number of signalling failures occurring on the network. In fact, there has been a steady decrease year-on-year. According to Network Rail, in 2011/12 there were 23,000 signalling failures on the network causing a delay of 100 minutes or more, in 2014/15 the figure was down to just over 20,000 and in 2016/17 it had reduced to 19,000. This is still a lot, as even one failure is one too many, but the trend continues downwards.
The Office of Rail and Road (ORR) is concerned about passenger train service performance in the North West and Central region in England and has, in January 2020, put Network Rail on a warning for its poor overall service, saying that “Network Rail’s performance in terms of its contribution to delays in this region remains a concern”. The train operating companies for this area include Northern and Trans Pennine Express, which have also been criticised for their poor performance.
However, signalling failures in this region continue to improve. The overall number of signalling failures in the North West and Central region was 14 per cent lower, year to date, in January 2020 compared to January 2019. Signal failures were nine per cent lower, level crossing failures 11 per cent lower, signalling power supply failures 24 per cent lower and track circuit failures 24 per cent lower. Telecoms failures contributing to signalling failures were down 25 per cent.
However, permanent way track faults were 14 per cent worse and traction power supply failures were 11 per cent worse, hence the ORR concern with the overall poor asset performance.
The ORR also looked at the cause of the recent poor performance of Trans Pennine Express (TPE) in the region and found it had been largely the result of train operations and not necessarily asset failures.
So how has the reduction in signalling failures been achieved? Improvements in the reliability of signals are due to the progressive introduction of LED signal heads over the last 10 to 15 years, although LED technology has caused some reliability problems, especially in the proving interface with systems designed around incandescent lamps.
Track circuit performance has improved with the introduction of moulded tail cables, the upgrading of troublesome equipment, upgrading older installations with duplicated tail cables, and master-class initiatives to share best practise and improve competency with maintaining insulated rail joints.
Predict and Prevent
‘Predict and prevent’, rather than ‘find and fix’, maintenance is the objective. Key to this is remote condition monitoring (RCM), which is an umbrella term for a number of remote monitoring strategies including points and track condition monitoring using analogue sensors or event monitoring of signalling control logic. These systems are used to monitor and report condition and defects so that action can be taken before failures occur.
Reliability-centred maintenance, on the other hand, links maintenance with usage and performance. It identifies historic maintenance tasks that cannot be demonstrated to be beneficial to asset performance, so they can be eliminated or, at least, performed less frequently. It also considers possible additional maintenance tasks and frequencies for assets that are used intensively or are of strategic importance. In simple terms, heavily used critical assets are given more attention than lightly used ones.
In-built resilience
There is still much to do before “signalling failure” is no longer heard on station PA announcements, news or social media. Even when a potential fault is identified by the maintainer, gaining track access on a busy railway which is operating without a problem can be a challenge, and, when faults do occur, getting the right staff to site can also be a problem in the busier parts of the country.
This is why new assets have to be designed so that they do not fail, with redundancy and resilience built in from day one. Nowadays, people expect everything they buy, be it a car, phone, washing machine or any consumer device, to work reliably out of the box. When buying new infrastructure, the railway demands and expects no less from the signalling supply industry.
New signalling systems are designed with far more resilience than previous ones, with diversity and redundancy built in. Processor-based systems with hot standby and double or triple redundancy are now available and in service, and which are able to have any failed critical components replaced with the systems still operational. The telecoms network now employs ‘packet routing’ internet protocol IP, which provides multiple connections for signalling and radio systems.
The reduction in the number of signalling asset failures can therefore be attributed to the large investment in new signalling assets, to targeted interventions and to the benefits stemming from remote condition monitoring that can identify problems before they become failures. Of course, one must also add the sheer hard work and attention to detail of all those involved.
A reason for the overall poor performance, and increased media reporting of the problems, is no doubt the greater number of train services on the network along with the large number of passengers on those trains. It only takes one signalling failure to delay a busy train and the internet is immediately full of tweets and other messages informing the world of yet another “signalling failure”!
