A crowd of people gathered outside Paddington station in an attempt to to be on the first Elizabeth line train when services begin at 6.30am.
There were around 300 people waiting at 6am for the new service, which will also run to and from Reading later in the year.
Colin Kelso, 18, travelled down from Glasgow for the opening. He was wearing a hoody emblazoned with "Purple train" on the front, in a nod to the line's colour scheme.
He told the PA news agency: "I want to get on the first train. I've always liked trains and have been keeping up to date with the project."
Danny McLaren, 21, from Edinburgh, arrived at Paddington at 1.30am.
He said: "We've known it will open for a while.
"It's a brand new railway. New technology. New trains.
"It's an epic day to experience it when it's brand new."
Reading commuters will not see all of the new line's benefits for another year.
For all the fanfare surrounding London’s newest train route, you would be forgiven for thinking it was finished – albeit four years later than originally planned.
Travelers boarding at Reading, Maidenhead and Slough will soon have access to more modern, less crowded trains, and some will have faster commutes, but a seamless transition into central or east London will still take until ‘no later than May 2023’, according to Crossrail.
Another passenger, Hakim Colclough, 24, from Chessington, Surrey, said: "This is a momentous occasion.
"It's a once-in-a-lifetime thing."
Another passenger at Paddington, Colin Farmer, 84, from Croydon, south-east London, arrived at 4.30am.
He said: "It's history. It's about time there were trains right through London without changing to the Underground.
"I'm very excited. We've been waiting long enough for it.
"It's a great achievement."
The crowd cheered and rushed forwards when the doors to Paddington Elizabeth line station opened at around 6.20am.
The first Elizabeth line train from Paddington through central London departed on time at 6.33am carrying hundreds of excited transport enthusiasts.
Another passenger, Mark Davis, 48, from Canary Wharf, south-east London, said: "We've been living with this for what feels like 10 years.
"We live in Canary Wharf, so it's a game-changer.
"It's the air-conditioning that makes the difference."
James Robert, 48, from Wigan, was on the train with his 11-year-old son Matthew.
Mr Robert said: "It's lovely and bright and fast.
"I wish we had trains like this in Wigan."
London mayor Sadiq Khan, who also travelled on the first train, said: "It's a landmark day.
"I'm excited. I'm like the little boy before Christmas."
He said the Elizabeth line is a "game-changer" that will "transform our city".
He added that the trains are "fit for a Queen" after her Majesty visited Paddington last week to mark the completion of the Crossrail project.
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