The sight of a steam train emerging from the tunnels of the Tube in a swirl of smoke makes for a slightly surreal experience.
But that is what happened as London Underground marked 150 years since the first underground train journey by taking a restored steam locomotive, which had hauled passenger carriages in the 19th Century, on a commemorative journey to retrace its original roots.
To the passengers on board and the spectators who caught a glimpse of it rushing past, it was history in action.
It evoked a by-gone era and hinted at what the underground was like in 1863 when it was run with steam engines.
The journey, along part of the original Metropolitan Line, was three years in the planning as the locomotive and carriages had to be restored to their former glory.
'Extra special day'
The train began its commemorative journey in Kensington Olympia station in west London.
The platform was packed - not only with the fortunate passengers who had their Willy Wonka-esque burgundy and gold tickets, but also with steam enthusiasts who had braved the bitter cold to catch a glimpse of this majestic train.
Jack Fitzsimmons, 27, got up at 05:30 GMT to leave his home in Southend-on-Sea, Essex, in order to see Met Locomotive No 1.
"I come up to London and see steam trains all the time but because this is a one off, it will be even more special then any others I've been to because it's such a rarity," he said.
"This is an extra special day even though it's absolutely freezing, it's totally worth it."
As the train pulled into the station, passengers and onlookers bounded towards the top of the platform to take photos.
Today, Met Locomotive No 1 was the star.
Once all the photos had been taken and passengers were aboard the train, the familiar sounds of its horn signalled that we were on our way.
As the train breezed through 12 stations, it was greeted by thousands of onlookers.
'It was magical'
Some stood with their mouths wide open in surprise while others clapped and cheered as they all took in the bizarre sight of a steam train emerging from a tunnel only to quickly disappear again.
Chugging along at a speed of 25mph - modern day Tube trains can reach 50mph - it headed through central London via the Circle line; taking in stations including Edgware Road, Baker Street, King's Cross and the Barbican before arriving into Moorgate on time at 10:30 GMT.
Tony Lyster, chairman of trustees for Buckinghamshire Railway Centre which owns the locomotive, reminisced afterwards.
"It was wonderful - the smell of the steam coming through the windows - we weren't allowed to open them but we opened it just a crack; the sound of the locomotive chuffing away through the tunnel, particularly on the run up of the hill towards Notting Hill Gate.
"It was magical - and to be in the old carriages as well."
Built in 1898, the steam engine was restored specially for the anniversary.
"It goes beyond expectations and it's so lovely to see her back in steam again.
"She's only been out for a couple of years (to be restored) but it's been tremendous," said Mr Lyster.
London Mayor Boris Johnson said: "This was really incredible because it was exactly as the Victorian underground traveller would have experienced it."
He added: "When it was built, it was an utterly revolutionary piece of technology and the idea of sending a train through a tunnel under the city - no one had thought of that before."
The person who had the daunting task of ensuring there was enough steam to drive the train was Oliver Furnell from the Forest of Dean.
The fireman, who ordinarily drives trains through heritage lines in the countryside, said there was no real difference in how to drive it, but the "environment that it's happening in, that's the unusual bit".
"It's quite surreal to be rolling through a modern Tube station and people seem to be pleased that you're there," he added.
"The steam and smoke has got nowhere to go to other than around the train and in the tunnel, so yes I suppose you get a better quantity of smell. It's a heightened experience."
Met Locomotive No 1 was due to make several further trips through the Underground on Sunday evening and again in a week's time.
However Sam Mullins, director of the London Transport Museum, hinted that this could happen more often.
"We will do more events like this this year and more into the future, here and on other railways," he said.
Two boys who would be eager to experience the train again are Henry Thomas, 12, and 13-year-old Sam Wild, who had travelled to Moorgate station from Bath just to catch a glimpse of the train in action.
"I really like the atmosphere around the steam train as everyone is really excited to see it," said Sam.
Henry added: "It was definitely worth it, it's been absolutely fantastic.
"It's always hard to explain whenever people ask me why I like steam trains. The smell, just being around it, it's just absolutely fantastic.
"It's a thing you can't explain."