They were carrying a piece of burning paper as a lamp and when the lit paper was dropped inside the bridge, the wood and tar inside the tubular structure caught fire. Strong wind and the tubular shape helped the fire spread along the bridge. The fire continued to burn for nine hours.
One of the youths later said they had been invited to a party by a girl but when the group got to the house they found her parents had gone out and locked it.
Instead of the party they decided to go down to the “tube” for a walk and after climbing a stile went into the structure for about 10 yards.
He told reporters in 1972: “We had never been there before and just wanted to see what it was like. We couldn’t see much just hear a noise from the girders.
“There was a page of a book on the floor and I had my lighter. I lit the paper and threw it behind one of the girders.”
The burning paper set light to tar on the wooden sleepers on the track. As the roof was wood the fire quickly spread, from the mainland side of the bridge towards Anglesey, aided by strong winds.
Fire crews from Bangor were first on the scene after the alarm was raised at 9.43pm and found the entrance tower well alight, with fire already spreading rapidly over the roof of the first span.
A former Bangor fire fighter said later the tube was “like a long chimney on its side with a massive draw”.
More appliances were requested and 11 appliances and more than 60 fire fighters fought the blaze but they were hampered by the wind and the inaccessible spot with low water pressure from hydrants.
The tar covering the tubes melted and began falling from the bridge, setting light to trees and undergrowth below.
The glow of the blaze could be seen from as far away as Holyhead and Llandudno and burnt fiercely for nine hours before firefighters succeeded in bringing it under control.
But Robert Stephenson’s bridge, which had stood for 120 years, was virtually destroyed, severing Anglesey’s rail link with the mainland.
An official report into the fire fighting operation revealed low water pressure hindered the firemen’s work.
Frank Hitchinson, the chief officer of Caernarvonshire ((CORR)) Fire Brigade, said there was no water hydrant near the bridge and the nearest water supply was the Menai Strait about 450 yards away and down gradients of one in three.
He said later the lack of a water hydrant was known to both fire services and British Railways (BR).
“The fire was on railway property and they looked after their own fire prevention arrangements. It would have been ideal to have a water hydrant near but we can’t interfere with railway property in that way,” he said.
Fire chiefs were not aware a “caretaker” – a man posted at both ends of the bridge to ensure no one made their way into the tubular bridge – had been withdrawn.
The report noted the “greatest difficulty in tackling the fire was lack of access to the underside of the common roof spanning both tubes, and the 10 feet gap between the tunes allowing the wind to assist spread of fire to the Anglesey side of the bridge”.
With the fire spreading rapidly across the bridge the Divisional Officer at the scene sent a further message to Fire Control at 10.44pm stating “Fire spreading rapidly. Request Anglesey Fire Brigade to send five pumps to the Anglesey side of the Tubular Bridge.”
Fed by high winds the fire was racing towards the Central tower and the Chief Officer of Caernarvonshire sent two of his appliances to assist the Anglesey firemen. It was hoped the island firefighters might stop the fire at the Central Tower.
The report added: “For a considerable time it appears the efforts of the Anglesey firemen would be successful, but the fire in the roof void was so severe that it broke out onto the surface of the roof behind the point where the fireman were operating, and the Chief Officer gave instructions to withdraw all firemen as there was great danger of lives being lost.”
Although fire-fighting operations continued through the night, the fire was not contained until the morning and firemen spent the day extinguishing fires in the hollow masonry towers and along the banks of the Menai Strait.
The Secretary of State for Wales – the Rt Hon George Thomas MP visited the scene to inspect the fire damage the following day and congratulated the fireman of both brigades ”for the brave and courageous manner in which they fought and tackled the fire”, and endeavoured to save this vital communications link between the mainland and the county of Anglesey.
Both fire brigades also received letters and message of congratulations from the then Prime Minister Harold Wilson, Lord Snowdon and other local authorities.
The teenagers who had gone onto the bridge were not prosecuted as it was deemed to be an accident.
“I wish we hadn’t gone to no party now,” the youth said ruefully.
Trains were running again between Anglesey and the mainland within 18 months.
It would be 10 years, however, before the new road deck above the railway opened.
Cllr Tom Jones, the Gwynedd Council highways chairman, speaking at the official opening of the road deck, said: “This bridge really is a case of ‘out of the ashes arose new life. For, by the sad event of the bridge getting accidentally burnt, something doubly good had been built instead.”
With traffic increasing, there are discussions concerning upgrading the Britannia Bridge once again.
There has been no decision so far, so perhaps three bridges will cross the Menai Strait in future.