“The two critical paths for the project remain software development for the signalling and train systems, and the complex assurance and handover process for the railway; both involve safety certification for the Elizabeth line. These must be done to the highest quality standards to ensure reliability of the railway from day one of passenger service.

“Crossrail Ltd will need further time to complete software development for the signalling and train systems and the safety approvals process for the railway. The Trial Running phase will begin at the earliest opportunity in 2020, this will be followed by testing of the operational railway to ensure it is safe and reliable.”
source= https://railway-news.com/crossrail-central-section-will-not-open-in-2020/