Crossrail has announced it is entering a “crucial” development phase as it doubles the number of test trains.
The process involves “simulating a timetabled service” on the railway by running eight trains through its central tunnels.
The project was initially due to be completed in December 2018 but has been delayed and is over budget.
It is not expected to open until the first half of 2022, when it will be known as the Elizabeth line.
The line will run from Berkshire to Essex via central London.
Crossrail Ltd chief executive Mark Wild said: “This is an incredibly important milestone for Crossrail to reach and puts us firmly on the journey to unlocking trial running next year.
“We are doing everything possible to deliver the Elizabeth line as safely and as quickly as we can, and we know that Londoners are relying on the capacity and connectivity the Elizabeth line will bring.
“This milestone also marks the incorporation of our Romford control room colleagues into the testing process, another sign that we are moving ever closer to our final goal of an operational railway.”
Crossrail Ltd described delivery of the scheme as being “in its complex final stages”, with “good progress” made on completing infrastructure work.
The project was given £825 million more Government funding earlier this month.
Its budget was set at £15.9 billion in 2007 but it is now expected to cost around £18 billion.