This article was published in IRSE News May 2021.

On 30 July 2020 Elizabeth line (formerly Crossrail) trains began to run in passenger service under ETCS Level 2 between Airport Junction and the London Heathrow Airport stations, about 7km. This was the first stage of the Network Rail London Paddington to Heathrow Airport ETCS L2 Overlay Project.

The 20km from the Paddington terminus to Airport Junction, where the branch to Heathrow diverges, is part of the main line from London to South Wales and the West of England. ETCS was to be applied to a 4-track mixed traffic railway with the retention of existing signalling controlled by computer based interlockings.

Existing train protection was provided by an ATP system based on the Belgian TBL system and by the British AWS and TPWS systems. TPWS however is not fitted on the Heathrow Airport branch. The line speed is up to 200km/h. Initially only rolling stock running to the airport is being equipped with ETCS.

This article sets out to describe some of the signalling principles decisions that the project had to address, including:

• Permissive movements
• Approach release of aspect
• Release speeds and overlaps
• Release of approach locking and overlap locking
• Signal alight proving
• Staff responsible mode
• ETCS/lineside signalling transitions
• Start of mission