Low Level 09 Signal Sighting Model

The benefits of virtual signal modelling

Years of study and analysis of hardware and logic have delivered very reliable signalling equipment. More recent events have highlighted the need to focus on signal location, sighting distances and the way in which signals are interpreted by drivers. This is challenging, as traditional techniques for choosing the positions of signals have become harder to use.

Safety regulations make it difficult for staff to gain access to the track, and increasing traffic means operators wish to minimise disruption. Route learning is difficult because a new layout cannot be driven until it is completed, and even then it may not be possible for a driver to pass along every possible route. The rail industry has thus had to devise new ways of establishing signal sighting, and communicate final designs to train drivers. An approach which has been used on Thameslink Programme uses a 3D computer model of the railway, which can be used to place signals by parametric design rules and then perform objective assessment of sighting, aiding subjective assessment within the real environment.

The role of signal sighting for Thameslink Programme

Thameslink  Programme was developed to deliver a major expansion of the cross-London Thameslink Service. It involved the provision of longer, more frequent trains, serving a more extensive network to deliver a substantial increase in passenger capacity through the centre of London.  A 16 day blockade was identified from Saturday 20 December 2014 to Monday 5 January 2015 for the LL09 staged works at London Bridge station.  Stage LL09 covered:

  • The removal of platform 8 & 9
  • Remodelling of the station throat
  • Temporary track slews
  • Construction of crossover
  • Installing of much of the final signalling infrastructure, including new lightweight straight post signal.

Using survey data, 3D track data and 3D signals provide a comprehensive data set for signal sighting. This seamless solution allows real-time positioning and movement of signal objects and signs, additionally allowing clash detection modelling. Virtual Modelling is an important component part of the construction phase of many schemes, allowing the model to be used throughout the entire life cycle of projects, from design through to Signal Sighting and on into Driver Training and Simulation.

Thameslink Programme has used a 3D computer model of the railway. It can be used to place signals by parametric design rules and then perform objective assessment of sighting, aiding subjective assessment within the real environment

Virtual railway model

The models are generally based on a number of datasets. A static element consists of the terrain, buildings and earthworks, which rarely change. There is also a semi-static model, comprising the rails, overhead line and other features that can be updated on an intermittent basis, requiring some data changes. Finally, there is a dynamic model, which holds the signals themselves and is automatically updated by the signal-sighting software as each signal is placed and design decisions evolve.

These elements combine to form a complete model of the railway, which can be analysed and viewed at will. The output can be generated in a variety of forms depending on the user’s needs, including drawings for design engineers wire line diagrams, static images and animations for external users and interactive viewers for subjective assessment.

The most important feature is that this should be a seamless process, using a single data model generally known as a virtual railway.

This requires a variety of software components:

  • Three-dimensional string modeller for terrain, track geometry, earthworks and overhead line
  • Solid modeller for structures and buildings
  • Rendering engine to create high-quality still images and animations
  • Signal placement and sighting tool
  • Viewer for quick visual assessment of the design
  • A simulation system for communicating the design to the drivers.

Sighting requirements

A number of different software tools are used to meet the signal-sighting requirements, including placing signals, performing calculations, creating ‘flight lines’ for the driver’s eye position as the train moves along the routes, and interaction with the viewer.

The placement tool allows the designer to create signal objects in the 3D model and configure them. The tool allocates an identity, composes the signal from components, places the signal in the model, and sets signal direction and dip. Once these have all been assigned, the signal can be placed into the model for immediate viewing.

The user can change the signal or any of its attributes at will, immediately seeing the effect in the viewer. It is simple to adjust the placing of the signal to avoid any problems such as read-through or obstruction by other equipment.

The performance calculations tool allows the user to perform intervisibility calculations based on the location and direction of signals, obstructions, and the driver’s eye flight line. The tool creates rays for line of sight, calculates obstructions and the 7 sec and 4 sec positions required for first glimpse and continuous sighting. It can also identify possible read-through locations, establish a reserved signal volume, and create signal-sighting forms. The line-of-sight calculations take into account the conicity of the signal lamp lens and the viewing angle for the driver’s eye, constrained by the viewing angle of the cab window.

Lines showing the rays from the signal to the driver’s eye position are generated in 3D and can be viewed in plan or wire-line perspective. Based on the 3D rays, a reserved volume can be established for the signal, into which no obstruction can be placed without reference to the signalling engineer. The final product of this process is the signal-sighting form, with the basic details for the signal filled in automatically.


Using survey data, 3D track data, 3D signals and scheme plans provide a comprehensive data set for signal sighting. This seamless solution helps assess reading distances of signals with precision in an interactive engineering model allowing designated Train and Freight Operating Company (TOC and FOC) representatives ultimate confidence in signing off required Signal Sighting assurance documentation.

Virtual modelling has reduced the need for site visits and assumptions in the suitability of proposed schemes ensuring a Safety by Design approach as well as adopting Network Rail’s policy of ‘Everyone Home Safe Everyday’.

About the author

Case study produced by Tina Cleland, Programme Engineer – Network Rail Infrastructure Projects, October 2018.

Further information

For more information on this Learning Legacy case study please email contact@thameslinkprogramme.co.uk