The sub-surface Circle, District, Hammersmith & City and Metropolitan lines are set to have a new signalling system in order to improve reliability and services.
From TfL press release
Reliability and customer information on sections of Circle and Hammersmith & City lines to improve from next year.
· New system will mean quicker, more frequent services on sub-surface lines from 2021
· 40% of Tube to benefit from huge improvements by 2023
· The transformation of London Underground's sub-surface lines has taken a significant step forward, with multiple test trains running on the brand new signalling system for the first time in London.