Work has begun to upgrade a signalling system on the Thameslink line as part of measures to improve reliability on one of the busiest parts of the rail network.

To help reduce delays, Network Rail engineers are installing a backup train detection system between London Blackfriars and Farringdon, so if one fails, a secondary system can take over.

Train detection systems play a vital role in keeping services running safely and on time by monitoring trains’ positions and movements. However, in recent years, a small number of faults with the current system have caused significant delays and disruption for customers travelling through central London.

When faults occur, trains must run at reduced frequency under strict safety conditions, meaning fewer services can pass through, and journeys can take longer or be cancelled.

The upgrade to a new dual detection system will see axle counters installed alongside existing track circuits, providing an additional layer of resilience so that trains can continue to run even if one system fails.

A track circuit is an electrical system that detects the absence of a train on a section of track. Axle counters perform a similar function but count the wheels of trains in and out of a section of track. Both systems help the signalling system know if it’s safe for another train to proceed.

Network Rail is also replacing the 20MW electrical substation between City Thameslink and Blackfriars station. This is the largest on the 750V third-rail network and is under heavy load due to the volume of trains passing through.

And following a successful trial, Network Rail and Thameslink are also looking to fit thermal imaging cameras permanently to the front of Thameslink trains to detect ‘heat spots’ and catch faults before they even happen.

The Thameslink ‘core’ route between London St Pancras International and Blackfriars is one of the most intensively used sections of railway in the UK, with up to 44 trains per hour on a two-track system connecting services from Kent, Sussex and the East Midlands through the heart of London.

With frequent trains and no diversionary routes available, even small faults can quickly cause disruption across the network, as far afield as Brighton and Cambridge or Horsham and Peterborough.

Louise Greaves, Network Rail’s Infrastructure Director said: “The heart of Thameslink, in central London, is one of the most critical sections of railway in the country, where even small issues can quickly have a big impact on thousands of passengers.

“By installing a dual detection system, upgrading the power supply and looking at the fitment of train cameras, we’re making a targeted investment in reliability and resilience. Two detection systems means that even if one fails, trains can keep moving, reducing disruption and improving journeys for passengers.”

‘Dual detection’ technology was successfully introduced north of Farringdon in 2020, where delays caused by track circuit failures have declined. Similar improvements are expected once the new system is in place between Blackfriars and Farringdon.

Following installation, the system will be tested over the autumn of 2026, with final commissioning planned by the end of the year.

The deployment of the dual train tracking system is unrelated to the recent trials of the radio signalling system, which is also being upgraded.