Crossrail is running test trains in London as it ramps up plans for a 2022 opening.
The new Elizabeth Line is operating four trains an hour as part of safety testing, which Crossrail is calling a "crucial milestone" in the £19bn rail link in the capital.
The trial run will be followed by tests of train stations and systems later this year, with Crossrail targeting next year for a full opening, four years behind the original schedule.
Test trains are now running along the length of the line, combining the new tunnelled underground sections with the overground stretches of the Great Western and Eastern mainlines, eventually linking Reading to Essex.
Recently re-elected London mayor Sadiq Khan said: "Safely delivering the Elizabeth Line as soon as possible is one of my top priorities, so I’m really pleased that trial running is now under way.
"The whole Crossrail team are doing all they can to get the railway open and ensure London and the wider South East can enjoy its many benefits sooner rather than later."
Mark Wild, Crossrail’s chief executive, added: "We have now started the trial running of trains on the Elizabeth Line and this is an incredibly significant moment.
"It marks the moment when our focus shifts to commissioning of the new railway and it puts us firmly on the path to trial operations and ultimately the opening of the Elizabeth Line.
"I want to thank everyone who has worked so hard over many years to get us to this point.
"There have been real challenges along the way but the start of trail running is an important milestone for the Elizabeth Line and for London."
Combining the three different stretches, which run on separate signalling systems, had been one of the main engineering difficulties Crossrail encountered as its budget spiralled upwards and repeated delays were announced from 2018.
The total estimated cost of the mass-transit train line was initially £14.8bn but rose to £18.2bn in 2020.
Originally planned to open in 2018, the project has continued to be delayed with 2022 now the target.
The onset of the COVID-19 pandemic caused further delays, with Crossrail forced to pause physical work on all sites.
Work did restart last year, but social distancing meant a maximum of 2,000 people were allowed on site – less than 50% of the pre-pandemic staffing levels.
Construction of the revamped Tottenham Court Road station, set to become one of London’s busiest interchanges, has now been completed.