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SIGNALLING & TELECOMS
DAVID BICKELL
Improving performance and capacity on the railway.
Conventional signalling and the European Train Control System (ETCS)
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eaders of Rail Engineer are well acquainted with the huge construction projects around the country, aiming to increase capacity by building additional platforms and/or providing grade separated junctions. Examples include Norton Bridge, Heathrow Airport junctions, London Bridge, Reading, and Peterborough.
These schemes are extremely costly. They involve massive and very visible civil engineering works that create a significantly enlarged footprint of the station or junction, accompanied by substantial, mostly behind-the-scenes, alterations to the signalling and telecommunications (S&T) infrastructure, not forgetting the extensive operational changes that Network Rail and the TOCs have to plan for and implement. With project costs coming under intense scrutiny, the Digital Railway is looking at options to deliver significant increases in performance and capacity as an alternative to the need for extensive remodelling works, which to a certain extent, are dictated by the design requirements of colour light signalling, as this article describes.
Limitations of multiple aspect signalling
Fig 1: Braking distance three-aspect signalling. Fig 2: Braking distance four-aspect signalling.
Colour light signalling began to be installed on main line railways from the 1920s, providing train drivers with vastly improved signal sighting compared with semaphore signals, and facilitating the introduction of centralised signalling control centres. However, colour light signals, in addition to indicating what we now call ‘movement authority’ (MA), also provide information to drivers as to the location at which braking should commence in
order not to exceed the limit of the current MA. This is achieved by a red signal being preceded by a yellow caution aspect in three-aspect areas, or a double yellow followed by a single yellow in four-aspect areas. It follows that the first caution signal must be positioned at least at braking distance from the signal displaying red. Distance ‘a’ (three-aspect signalling) and ‘x+y’ (four-aspect signalling) must be a minimum of braking distance. In four-aspect areas, some flexibility in signal positioning is permitted within the stipulation that the distance between the single yellow aspect and the red aspect (‘y’) shall be no less than one-third of the actual signalling braking distance between the double yellow aspect and the red aspect (x+y). To get an appreciation for the signal spacing distances involved, on a level gradient, the braking distance for a line speed of 125 mph is 2,054 metres. The full requirements are detailed in Railway Group Standard GK/RT0075 and it is incumbent upon the signal design engineer to ensure that the position of lineside signals shall be compatible with the braking performance of rolling stock so that trains moving at the permissible speed can stop within the actual signalling braking distance. Put simply, when creating a signalling scheme plan, firstly signals are positioned to protect junctions, and control movements starting from platforms and sidings. The preceding signals must be positioned to provide the necessary braking distance. It is these significant distances involved that may impact upon performance and capacity, as will be outlined in a moment.
European Train Control System (ETCS) ETCS is not a complete signalling system in its own right but provides an interface between signalling trackside infrastructure and individual trains. The Driver Machine Interface (DMI) in the driving cab displays the distance for which the train is authorised to travel, and the maximum speed allowed. If the onboard computer predicts that these values are likely to be exceeded, the system intervenes to safeguard operation of the train.
Rail Engineer | Issue 161 | March 2018