The Elizabeth Line, London’s long overdue cross-town rail link, has begun trial operations and is expected to open to the public in the first half of next year.

The train service, originally dubbed Crossrail, began test runs on Saturday, Transport for London said in a statement on Monday.

Construction work on Europe’s biggest infrastructure project began in 2009 and was originally slated to be completed before the end of 2018. The initial opening will involve just the central section of the line, which connects Paddington to the west of central London and Abbey Wood on the city’s eastern fringes.

The outer reaches of the line—linking Reading, about 45 miles (72 kilometers) west of London, and Shenfield, 30 miles east—will initially run as separate services before being connected to the central section no later than May 2023. The service will also include a link to Heathrow Airport.

The final phase of trials, involving mock evacuations with thousands of staff and volunteers, will begin in 2022. A final opening date will be announced once these tests are completed.

“The Elizabeth line will help transform travel in London and the South East, dramatically improving transport links, supporting regeneration and boosting our economic recovery,” Mayor of London Sadiq Khan said in the statement.