Providing the north-south axis to Elizabeth Line's east-west route through London, is Thameslink, a £6bn nine year refurbishment that completed in 2018.

Stretching from the south coast of England, through London’s suburbs and central districts, and as far north as Kings Lynn by the Norfolk coast, the Thameslink programme brought new, longer trains, resolved some notorious network bottlenecks, and delivered major improvements to key stations, such as Farringdon, Blackfriars and London Bridge, amongst others.

An upgrade of existing infrastructure (some tunnel sections date back to the 1860s) rather than a new build, Thameslink’s transformation was a gradual, incremental process that had to be constructed amidst the capital's functioning network ("like trying to conduct open heart surgery with the patient going about their daily routines", equated one commentator)

ETCS signalling in the Thameslink ‘Core’

Central to the entire programme is the ‘Core’ — a short, double-track section between Blackfriars and St Pancras through which all cross-city services pass. With a new signalling design to cater for up to 24 trains per hour (pervious capacity was 16tph) this section supports service frequencies more akin to that of a metro. And herein lied a major technical challenge.

Most metro systems under construction or refurbishment today would turn to Communications Based Train Control (CBTC) to manage the level of traffic intended for the Core (as, indeed, Crossrail will for its own central section). This is because CBTC is designed for the stop-start nature of metros, using automated control of train movements to allow shorter headways between services.

But as a Network Rail project — and consistent with their commitment to adopt train control technology championed by the EU to bring consistency across Member States — Thameslink uses  European Train Control System (ETCS) as its standard for signalling. This is despite preconceptions that ETCS is optimized for regional and national mainline railways and not a metro-style operation.

However, Thameslink has developed the concept of an ETCS railway overlaid on a conventional signalling system to deliver a ‘metro style’ operation. The high capacity is achieved through two new features:

• ETCS allowing the implementation of shorter block sections in a conventionally signalled area, in particular around stations
• The first use of an Automatic Train Operation (ATO) with ETCS on the mainline railway, to allow consistent train driving

Furthermore, by retaining the conventional signalling, a fall back system will be present in the event of ETCS failure.