Vehicle Track Interface

London Underground

Vehicle Track Interface

London Underground’s 4 Lines Modernisation (4LM) is a £5.4bn programme to upgrade some of the oldest lines with new trains and the introduction of Automatic Train Control (ATC).

To achieve the best possible service capacity and performance from the upgrade to an ATC railway, it is essential that optimum operating speeds are used, especially around the critical junction areas of the network, such as at termini, reversing sidings and where trains run across flat junctions.

CPC Role

London Underground wished to understand the optimum safe speed at which it is appropriate modern ATC should be able to operate across the lines and specifically across the critical points and switches. CPC Systems was engaged to provide leadership across disciplines to evaluate the constraints and produce a proposed Safety Case for the setting of operating speeds. This included:

  • The basis for the setting of track speeds in terms of derailment risk, passenger comfort, track condition and gauge clearance
  • The typical speed performance profile of a manually operated intermittent speed supervision railway relative to a continuously speed monitored Automatic Train Control system
  • The credible level of overspeeds compared to the operational set speed that could be projected for an ATC railway, both for normal service operation and in the worst case, with reference to the implications of such overspeeds drawing on the findings of the way track speeds are set

This activity drew upon the team’s experience of the key junction capacity areas, track formations and configurations, experience for derailment risk factors, previous ATC applications on London Underground, detailed knowledge of CBTC Safe Braking Requirements defined in IEEE 1474.1 and expertise in defining and justifying associated safety cases.

Commission Delivery

CPC Systems set out the basis for developing a proposed Safety Case for the setting of speeds across the key track areas which was developed and iterated through a series of meetings and engagement with key stakeholders, such as the various design authorities. The proposed Safety Case was then populated through a review of various aspects of TfL’s operations and infrastructure, including:

  • Assessment of the derailment risk associated with the points and switches at a series of junctions that are critical for future capacity growth
  • Assessment of compliance with current speed restrictions under manual operation,
  • Evaluation of existing ATC railways compliance with command speeds,
  • Review of Seltrac S40 Safe Braking Model applied to existing lines and the proposed model for the current 4LM upgrade.
  • Creation of a fault tree for a traction breakaway event, one of the limiting factors when determining maximal permissible speed on an ATC railway.

Based on the analysis and supporting evidence contributed by TfL, CPC Systems created a safety justification and proposed procedure for the definition of operating speeds that will provide significant performance improvements across complex areas such as flat junctions.

The large-scale review of service data to assess manual operations compliance with current speed restrictions required CPC Systems to develop specialised software to assimilate large datasets and compile velocity profiles for manually-driven trains on the 4LM network. The results of this comparison have highlighted the positive impact that implementation of an ATC system can have on the available capacity of a manually-driven railway, by utilizing the benefit of the ATC’s continuous and repeatable speed control.

Unique Added Value

CPC Systems were able to influence key stakeholders in a positive way that allowed an appropriate proposed Safety Case to be produced, supported by objective evidence. Having obtained the train logs in raw data form, with limited guidance on methods for its interpretation, the CPC team rapidly generated an interpretation tool capable of processing a huge amount of input data and plotting alongside the reference curves. Analysing the data by traditional means would have introduced a long lead time on reaching similar conclusions. This is an ability CPC have similarly exhibited on other commissions, demonstrating an agile approach to new systems and technical solutions.

Commission Results

This commission completed in April 2017 with the issuing of a proposed Safety Case, with supporting evidence and reports for the key conclusions. This is being progressed to the next stage of benefit realisation through application of the process within the proposed Safety Case to key junctions.

The analysis tool developed as part of the work has provided TfL with an insight into their railway operations, ultimately leading to a resolution not previously thought possible, in addition to the analysis of manual driving styles. The tool has since been used to compare planned and achieved runtimes, progression to evaluation of future estimated runtimes to ensure the 4LM upgrade will provide the planned capacity improvements. CPC Systems is also investigating the use of the tool to identify hot spots of poor adhesion and on board train faults prior to service affecting failures occurring in revenue operation.

Project Overview


Systems management


Transport & Infrastructure


2016 - 2017