These days we are often encouraged to think disruptively. The phrase is used to suggest that if we change the way we do things we will get better products and processes, often at lower cost. Is moving to European Train Control System (ETCS) one such example?
Moving from a lineside signalling system to an in-cab control system is certainly a big change. It is potentially disruptive to several areas of railway operation and engineering and therefore needs to be approached carefully to ensure it is completed successfully. It also has the potential to bring several substantial benefits to the railway.
Constraints of lineside signals
There are three fundamental constraints arising from lineside signals.
The first is that signals are in fixed positions. Since all train drivers must have time to read the message a signal is giving and then control the train appropriately, the distance between signals must be adequate to manage the train with the longest braking distance. This generates a fundamental compromise on signalling scheme design on a mixed traffic railway.
The second is the need to understand the human factors issues that may influence a driver’s response to a signal. Because of the nature of a train, responding to a misunderstanding will occasionally come too late to prevent an incident or, in the worst case, an accident. Thus, it becomes critical to ensure the meaning of a particular signal is clearly understood so the right response occurs.
Signals are therefore placed in clearly visible locations, at approximately equal intervals, and on multiple track railways in parallel locations in an attempt to ensure the correct signal is read. There is also the constraint of how many different aspects a signal can display which is, to some extent, limited by colours that can be easily distinguished several hundred metres away. Indeed, most railways also have systems to ensure the driver has their attention drawn to an approaching signal, and sometimes speed limits. In Britain it is the Automatic Warning System (AWS) system.
Finally, because signalling equipment is distributed widely across the network, power and communications equipment need to be provided to enable the equipment to function. This adds both capital cost in its provision and ongoing service costs keeping it maintained and functional.
It can therefore be seen that while lineside railway signalling is fundamentally about allowing trains to move safely across the network, there are several constraining factors that add cost and deliver a sub-optimal result.
Some of these constraints, especially those related to human factors, could be ameliorated by the use of a comprehensive Automatic Train Protection (ATP) system. If, however, lineside signals are retained it becomes just another added element of the system with further failure risk and very limited benefit other than improved safety. Remember, train safety on the railway – at least in Britain – is already very good.
Cab signalling
Using cab signalling, where the train knows its own braking performance, removes the first compromise. The infrastructure now only needs to tell the train the location at which it must stop. The train then informs the driver when its speed reaches the braking zone and, should the driver fail to react, the ATP function cuts in and initiates the brake response. Thus, compromises on maximum speed and interval between signals are removed. So are the constraints around signal aspects meaning in a few places, especially converging junctions, a joining train can be allowed to start much sooner after another train has passed.
In addition, with cab signalling constantly advising the driver of the maximum safe speed for that train, the human factors issues associated with lineside signals are much reduced. That is not to say there are no new human factors challenges to be considered, but with the support of ATP many of them are of a very different form.
Because there is only a need to inform the train of the stopping location the amount of lineside equipment can be substantially reduced. On mainlines with frequent traffic, section lengths may not change very much because of the need to keep following trains moving. On less densely used lines the normal signal spacing could be changed to suit the headway requirements, or those required to meet operational recovery needs, and on rural routes only the essential stopping places need to have any equipment at all. This is especially true with axle counter-based train detection.
With that background let us discuss the challenges and opportunities in more detail.
Challenges
The first challenge is train fitment, closely followed by driver training.
To be able to remove lineside signals, every train permitted to run along the route in open traffic, i.e. not under possession, must be fitted. Many railway assets are long lived, and this especially applies to rolling stock. Until a cab signalling system becomes universal there will inevitably be vehicles that need retro-fitment.
This is a multi-dimensional problem:
- Where will the equipment be fitted? Does that comply with all the system design criteria such as distance from the front end?
- Can the drivers desk be modified such that the driver can see and use the new cab display in all likely lighting conditions?
- The cab signalling system needs to know position and speed at any instance. Are suitable interfaces to tachometry systems available or can they be made available?
- Where can the necessary radio communications equipment and antenna be mounted? (Although with all trains now required to have a radio this is a relatively small problem.)
- How will the interface to the braking system for the ATP function be implemented?
- What disturbance to existing equipment is likely to facilitate this installation?
Those are just the technical questions. What about the commercial concerns of a vehicle being out of service? How long? What post fitment testing is required? Will reliability be impacted?
It is hardly surprising that retro-fitment costs are extremely high. Unsurprisingly, the trend is to a first of class fitment model to iron out the fitment arrangement and prove a satisfactory result before rolling the fitment across similar vehicles. That may not be the end of the challenge because, as we know, vehicles of the same class are not necessarily identical especially if they have a few years life under their belt. But once cab signalling is the standard fitment at build, it is both a relatively marginal cost compared to a new locomotive, even more so for a multiple unit train, and will be in a competitive marketplace compared to retro-fitment which has a very limited market place for each type of vehicle.
At the top of the operating tree is driver training. This is a substantial change to a driver’s normal working environment. Instead of looking for signals they are now required to monitor a display in the cab and respond to the prompts it provides while still keeping a close eye on the outside world for conditions or events that are not reflected in the signalling system – trespassers and trees come to mind. But here we also need to factor in frequency of use. The training can only be done when a driver is likely to use the new skill, otherwise that skill will be degraded or even lost.
However, that is not the only operational change that is created. How do platform staff know the train is able to proceed and thus the doors need to be closed? There is no proceed signal at the end of the platform. Even deeper into the operating organisation, and with reference to the earlier comment, there will be a change to how operators decide the signalling scheme design they need. Does this require a new or at least changed skill set to define the real operating parameters for the line?
We can then move on to rolling stock engineers who will need to diagnose and fix any faults with new and complex equipment. But even before that happens, how do we define the brake performance of a train and what safety margins are we going to employ? Do we have suitable current data available, or do we need to reorganise our braking models? Our accompanying feature “ETCS Implementation issues” explains how a train’s braking curve is input into the ETCS system and the issues associated with doing that for freight and other locomotive-hauled trains.
No easy solution
This is a double-edged problem because we need to define both a service brake performance and an ‘emergency’ brake performance and, if the latter takes longer to stop the train than that of the service brake, it will dominate and cause both human factors and operational challenges. This is especially a challenge for mixed formation trains such as freight or charter traffic. It also challenges the definition of an emergency brake as the one with the shortest stopping distance is not necessarily the one that is ultimately most reliable.
The signal engineer also has some major changes to consider and resolve. Fundamentally much of the signalling becomes ensuring a safe route is properly set for each train and that there are no or extremely limited opportunities for another vehicle to make an incursion into that route. But they also need to ensure complete details about the topography of the route, especially gradients, and that permitted speeds on every section of the route for each class of train are captured and stored, ready for transmission to the train as part of the movement authority. There is also likely to be a need to relearn the optimal sectional layout to achieve the desired headway for the traffic proposed on that route.
We then come to the electrification engineer. He gets one major benefit in not being required to consider signal sighting when positioning OHL equipment. The compensation is much more discussion to ensure stopping locations do not end up with the train gapped (on third rail) or being too close to a neutral or isolating section on overhead line.
The other feature of early implementations of ETCS is the testing. The current testing regime is proving very disruptive to railway operation. Better testing regimes are possible but one needs to gain confidence they are secure and suitable. Perhaps the initial disruption is understandable given the implications for safe travel if something is wrong.
While this is only a partial list, it illustrates that moving to a cab signalling system such as ETCS can be disruptive and we need to understand these challenges early in any roll out of the system.
Benefits
Having highlighted some of the reasons why it is so challenging to get ETCS functional on the railway we do need to appreciate the opportunities it brings.
The first and major benefit is it releases signalling or train control system design from the constraints of colour light signals on posts beside the railway. There is no longer a need to place signals according to restrictive standards that are there to ensure drivers know exactly how to react to the aspect being displayed. Sections can be much shorter where this provides a benefit such as at converging junctions or on approach to stations where occasional trains stop such as Stevenage or Grantham. Similarly, on multi-track sections of line would there be benefit having different stopping points to suit the dominant traffic?
There is of course a significant cost saving in not providing the signal and associated support structure or a power feed and interface to the interlocking. The absence of much lineside equipment will simplify ongoing maintenance, and should significantly reduce the failure rate and speed up return to service because of reduced travel time.
The signalling interlocking is significantly simplified because it fundamentally only needs to provide a proven safe path for the train. It no longer needs the logic to prove a suitable aspect is displayed on the signal and especially does not need the logic to manage junction signal aspect release as practiced in the UK. Aspect release at junctions has developed following overspeed incidents through divergent junctions and is essentially addressing a human factors problem but recent events, such as Spital Junction and Grantham to mention just two, show this is far from secure. Such overspeed events will be managed by ETCS and it will also enable emergency speed restrictions to be quickly applied without staff needing to go trackside.
These features will, in time, give confidence in the application of ETCS and will result in a substantial reduction in the design, testing and implementation cost of the interlocking.
On more lightly used parts of the network the volume of signalling equipment could be reduced. There are many branch lines that carry a moderate amount of traffic, and these are often signalled with regular three or sometimes four aspect signals. The frequency of signals is partly a response to the need to ensure they meet the human factors requirement for the driver to make very similar responses to every signal to avoid a misunderstanding. A signal should not result in an over-braked distance to the stop location – that may result in a potential headway that is less than is genuinely required.
Many, of these routes may well be able to have fewer signal sections using cab signalling thus reducing the overall Signal Equivalent Units (SEU) on the network generating a further total cost reduction. With a current SEU costing almost £0.5 million this could quickly become a sizeable sum. On even more rural routes, especially single lines, all signalling equipment can be placed at locations where it is needed, normally near the passing loops. The concentration of the signalling equipment in local areas will reduce the need for long power feeding arrangements, saving further money – something that has already been demonstrated by RETB.
Opportunities
Further opportunities are the relative ease with which an additional stopping location can be added to the system, perhaps to protect a new freight siding or a new station. Often adding such a facility to current signalling requires the relocation of several signals on either side to maintain the previously mentioned interval between signals. This can make such projects unaffordable. With cab signalling this is not a problem, a new end of authority can be relatively simply inserted. It will also be possible to remove the current embargo on speeds above 125mph which are driven by concerns about seeing and responding to lineside signals. However other trackside equipment may need to be uprated should this happen.
There may also be opportunities to extract train location and speed data from the Radio Block Centre (RBC). The RBC sits alongside the interlocking and holds the infrastructure data such as gradient and the maximum speed profiles for different types of train. It is responsible for transmitting the movement authority by radio to the train, including this additional data, having had the route which is set confirmed by the interlocking.
The RBC offers opportunities to enhance level crossing function and safety especially for the lightly used crossings which are currently User Worked possibly with warning systems but also for Automatic Half Barriers where train arrival times can pose a constraint if there is a significant difference in approach speed. Such data may also provide more nuanced input to traffic management systems enabling better junction optimisation. No doubt others will see further opportunities as they become familiar with cab signalling in much the way that standard colour light signalling today is not the same as it was when first implemented.
You will notice I have not mentioned increases in capacity. Fundamentally, capacity on a mixed traffic railway is determined by the mix of trains and stopping patterns not the signalling system. Yes, small gains may be possible, for example at converging junctions where the second train can be released sooner, but these gains are more likely to improve robustness in the timetable rather than increase the total number of trains that can operate. This is different to a metro railway with common rolling stock and stopping patterns, where more paths can be created.
In conclusion, I therefore suggest that we should expect some significant disruption and cost associated with the initial applications of cab signalling or, to be specific, ETCS, but, if we work hard, we can reap the future benefits it will offer once it becomes part of the day-to-day functioning of the railway.
Image credit: Network Rail
