An Elizabeth line train has made a journey through the tunnels of south-east London for the first time.
It comes days after the first Elizabeth line train ran from Paddington for tests.
The line, named in honour of the Queen, is expected to open in phases just before Christmas with passengers set to use 10 new state of the art stations across London and the south east.
“The maiden journey of a brand new train through the Elizabeth line tunnels is a milestone moment in one of the most ambitious infrastructure projects ever undertaken in the UK,” Mr Johnson said.
Testing on lines in south-east London will now continue until the line opens in stages from December this year.
London Mayor Sadiq Khan added: “This is a hugely exciting moment in the construction of the Elizabeth line.
“When it opens later this year, the Elizabeth line is going to transform travel across London and the South East, helping to improve the lives of millions of Londoners, stimulating much needed housing development across the capital, and providing a huge boost to our economy.”
The train, which was driven at low speeds, was built by Bombardier engineers in Derby.
More than 200 million passengers will be carried through 26 miles of tunnels on the Elizabeth line every year, adding 10 per cent to central London’s rail capacity.
The first stage of the line’s opening will see Paddington’s new Elizabeth Station open and link with Abbey Wood via central London.
The first Elizabeth line trains came into service in east London between Liverpool Street and Shenfield in June last year.
And its first Tube roundels were recently installed at stations across the capital, nine years after construction first began.
The £14.8 billion rail line will run from Reading and Heathrow in the west, to Shenfield and Abbey Wood in the east, slashing journey times into central London.