Rail Engineer has devoted many column inches to ETCS but mostly about its cost and deployment issues. ETCS is much more than a signalling system as it requires information about train formation, loads, and other characteristics. Inevitably, for a new system, some of these requirements might conflict with current national practice. It has been suggested that national practice should be changed in order to deliver the benefits of the inter-operable system. However, this is easier said than done, and resolving these issues takes time and money which is one reason why the first installations might cost more than the production run.
Some of those issues discussed here, if not addressed, could hamper operation of some types of train and could lead to some classes of locomotive being prohibited altogether. Partly this is as a result of Great Britain’s flexibility in what vehicles it allows to run where and partly because important aspects of GB practice were not incorporated into the standards/specifications. The risk is that lowest common denominator default values could be used, leading to a significant reduction in network capacity compared with the current lineside signalled railway with its rules and signage.
Rail Engineer understands that a paper outlining the issues was presented to the Office of Rail and Road (ORR) in early 2019. These are system issues that need to be captured in ETCS but cannot be resolved by signalling engineers alone. Network Rail and the ORR were asked for comment. Network Rail gave some useful feedback which is reproduced in full.
The issues are:
- Braking values for various train formations.
- Axle load categorisation for bridge loading.
- Implementation of differential speed limits.
- Cant deficiency rounding.
Cant deficiency refers to the difference between the actual cant (or superelevation) of the track and the ideal cant needed to balance the centrifugal force of a train traveling through a curve at a specific speed.
When a train travels faster than the speed for which the cant is designed, it experiences a cant deficiency, meaning the track is not tilted enough to counter the centrifugal force, leading to a lateral force pushing the train outwards.
Trains are designed and approved for particular maximum cant deficiency values, depending on their intended operation.
Braking values
ETCS onboard equipment needs to know the braking capability of each train. There are two means of entering braking data into the ETCS Driver Machine Interface (DMI). The first, Gamma, is a series of data generally applicable to fixed formation trains. The data is pre-loaded into the onboard equipment, and the driver selects (or the train could select automatically) based on the formation, e.g., four-car, eight-car. For GB application, Gamma is the proposed braking data format for multiple units (MU). However, for most existing GB MUs, the required Gamma data is not held on the industry database known as R2 and will need to be determined from design data/test train results. More recent MUs should have the Gamma value supplied by the manufacturer.
The other method is called Lambda. This is used to define braking capability for loco-hauled trains, both passenger and freight. The problem here is that the braking performance of GB domestic vehicles is not assessed against the UIC Leaflet 544-1: instead, Railway Group Standard GMRT2045 is used. As a result, R2 does not hold the ‘Brake Weight’ values required to determine the Lambda value. Even if R2 did hold the right information, TOPS (one of the oldest computer systems still in front line use) does not have the functionality to generate the correct Lambda value applicable to a particular train formation, for the driver to enter into the ETCS DMI.
Early proposals were to use a default Lambda value for all freight trains irrespective of actual formation/braking capability. This would inevitably slow the trains and consequently reduce the number of available paths.
Network Rail agreed with the above assessment and reported that:
“The sheer magnitude of variation outside of fixed formation MUs is something that ECDP and the wider community in the UK has recognised. Since the start of the decade the programme has been looking at this issue and the need for a robust and safe mechanism to input the data into the DMI and forms part of the Train Data Entry project.
“This is now well developed and will initially require a manual check of data, but progress is being made on using an app-based solution to provide the data to the driver. The use of a ‘Consisting app’ is already part of the freight scene in the UK and the mathematics to draw data from R2 to populate this activity is understood. R2 is not in all cases up to date, but for the vast majority of more modern wagons, this is an admin task from available data. For some older types, or those where no operational need pertains, a default value can be used. Similarly, for heritage operations there is not as much variety as in the freight space meaning a similar solution, possible a simplified version, will also be effective.
“This provision will enable optimisation of train performance and pathway allocation as ETCS rolls out across the network. ECDP as the pioneering programme recognises that until we have various OEM fitments, operating companies and train types in use on ECML, the full optimisation will not be possible. A key phase of development is that theoretically important calculations and standards are then optimised on the real railway to ensure that the full benefits of ETCS are delivered.”
Axle load categorisation
ETCS uses load categories as defined in EN15528 Railway applications – Line categories for managing the interface between load limits of vehicles and infrastructure. This does not map directly to the route availability (RA) categories used in GB. It is relatively straightforward to determine the EN 15528 load category for a train but is said to be a 20-year task to re-assess all structures (viaducts, bridges, culverts etc., on all lines, although it is understood that this task has started. Simplistic conversion can lead to permission to operate over tracks not currently permitted and vice versa.
GB has permission in the Infrastructure TSI (now NTSN) to continue using the RA method for assessment of compatibility for trains and infrastructure, but this permission was not included in the Command and Control System TSI. Thus all axle load related information used within ERTMS is categorised according to the EN15528 load categories and drivers are required to enter the EN15528 load category of the train during data entry.
As well as axle-load-related speed restrictions, the ERTMS route suitability function includes axle load as one of the factors against which route compatibility is assessed. This is again defined according to the EN15528 values. Therefore, until the infrastructure is re-assessed, this element of the ERTMS route suitability functionality will not be available for use in GB.
It is expected that a marginal additional cost would be incurred if structures are re-assessed to EN15528 as well as RA capability as part of the existing assessment schedules (hence the 20-year timeline). However, these assessment schedules are unlikely to match the ERTMS implementation schedule. There are potentially large cost, resource, and schedule implications if the structures have to be re-assessed outside the current scheduled structure assessments.
It should also be noted that the EN15528 method is not suited to some GB vehicle types, such as three-axle bogie locomotives (i.e., classes 37, 56, 60, 66, 69, 70, 92, and 99, and various heritage locomotives) whose capability could be dramatically reduced if the EN system is directly applied.
Network Rail reported that:
“The alignment of European categories to traditional UK Route Availability categories has been perceived as more difficult over recent times, but the strategy now being developed is to use the axle weight categories in the ETCS system as planned and link this data to RA categories. The trackside speed curves within the RBC will reflect the RA speed curves and therefore if the RA category is entered on the onboard system by the driver, then the appropriate RA speed curve in the RBC will be selected. There will then be a translation of RA to axle weight category available to the driver (lookup table initially and potentially input directly) so that each consist can have the appropriate RA curve.
“This minor adjustment is in development and would avoid a major change to bridge assessments and any loss of capability by various vehicle types.
“In the interim, traditional RT3973 forms [Advice to Train Crews – Conveyance of Exceptional Loads] will still be used for this as the development continues to automate parts of the process and provide the protection inbuilt in ETCS and will continue to be used for other operational purposes.”
For readers unfamiliar with the term RBC, this is the Radio Block Centre which is a trackside device for a particular section of track which holds infrastructure data such as trackside speed curves. It also communicates with interlocking and then uses all this information to issue trains with a movement authority via the GSM-R radio.
Rail Engineer understands that Network Rail’s statement above represents a transitional solution suitable for use on ECDP and potentially nationally, while a permanent solution is developed for national use. In addition, Network Rail is well aware of the potential conflict between the assessment method for EN15528 and its impact on use of three-axle bogie locomotives on certain structures and is developing a solution that will, at least, maintain current capacity.
Differential speed limits
Currently many lines have several differential speed limits. For example, parts of the West Coast Main Line fast tracks have three: 125mph for tilting trains fitted with Tilt Authorisation and Speed Supervision; 110mph for non-tilt multiple units capable of this speed; and 100mph for everything else. There are other examples applicable to certain types of train operating in certain places.
These categories are not catered for in ETCS so the default position would be for all trains to run at the lowest of the signed speeds at any location, with a significant reduction in capacity and adverse effects on journey times and rolling stock utilisation.
There have been proposals to deal with this issue. In summary, ETCS trackside would send a speed profile to the train which may consist of one or multiple speeds at any location. Where more than one speed is sent, each speed would be tagged effectively with the types of train that could use that speed. Each train type would have a set of data pre-programmed which the on-board uses to select the appropriate speed profiles which apply to it at any location, with the default value being the lowest if in doubt or in the absence of any other data. The ETCS system has limited criteria for differential speeds so some of the current GB flexibility cannot be included in this way.
ETCS does allow for the definition of different speeds depending on various train properties. This includes specified cant deficiency categories (see panel). However, the values defined for ETCS omit the GB values: 75mm (mainly freight; 90mm (some freight but mainly passenger); 110mm (passenger);185mm and 265mm (tilting trains). The default response would be to round down the cant deficiency value leading to a speed reduction of 2mph to 5mph, further impacting on journey time/capacity.
The alternative would be to move up to the next higher cant deficiency value. While some of the speed increases could probably be accommodated safely by vehicles, any speed increase at a specific location would require the gauge clearance for all vehicles to be checked which might identify gauge infringements or additional inspection requirements.
The speed reductions involved are quite small but could apply at many locations and the impact of these variations would need to be evaluated by modelling, but it’s just another factor that might slow trains.
Network Rail reported that:
“ECDP’s initial deployment for the training and migration phase will not include differential speed capability. This is being managed to avoid restrictive operational issues by Network Rail as an interim measure. From the first signals away area on ECDP (Biggleswade to Fletton), just a few years away, differential speeds for permanent speed restrictions, temporary speed restrictions, and emergency speed restrictions will be provided by an upgrade of the RBC capability.
“That functionality will then be retrofitted to the migration area from Welwyn to Hitchin in due course. As we look to deploy ETCS more widely, the key consideration during design for differential speeds will be in understanding the purpose of the restriction and matching the profile as closely as possible to one of the applicable ETCS categories and assessing the impact of any change from today’s working in so doing.”
Conclusion
It is good to see that ECDP recognises the issues and has ways of dealing with some of them, but it does highlight that the implementation of ETCS – fundamentally a speed signalling system compared with UK’s current route signalling approach – impacts on a lot more than is usually covered by signalling design. This is a business change programme that fundamentally affects how train drivers work.
In particular, freight train drivers have to enter data into the system, representing a risk of incorrect data entry. It is good to see that ECDP recognises this risk and acknowledges the need for “a robust and safe mechanism to input the data into the Driver Machine Interface (DMI)”.
As Network Rail acknowledges, there are still issues to resolve and no doubt Rail Engineer will return to this topic.
Lead image photo credit: istockphoto.com/kodachrome25