Engineers have tested one of the UK’s most intriguing railway legends: that the rising sun shines through the Box tunnel near Bath on the birthday of the 19th-century genius who created the line.

For many years, railway enthusiasts and mathematicians have argued over whether Isambard Kingdom Brunel, the creator of the Great Western mainline, did design the two-mile tunnel with his own birthday in mind.

On Sunday – 211 years after Brunel’s birth – the line was shut because of upgrade work, providing Great Western Railway and Network Rail with a rare chance to observe whether the sun really does shine through the length of the tunnel on 9 April.

Teams positioned themselves at the eastern and western ends of the tunnel and waited for the sun to rise on one of the brightest days of the year.

Communications manager Paul Gentleman, who was at the eastern end, said he was treated to a spectacular sight. “When you look from the east portal, the cutting provides a lovely V-shape,” he said. “The sun rose from the left and was shining directly down the tunnel. We couldn’t see how far.

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“It was fascinating to watch the sun as it nestled in the centre of that V as it rose – quite an astounding sight. When you look at it from that east portal you got the feeling that effect was definitely part of the design.”

At the western end, it was not quite as striking. Matthew Golton, commercial development director at GWR, said: “We could see the sun had risen but we weren’t getting full-on sunshine through the tunnel.”

Facebook Twitter Pinterest Light beaming through Brunel’s Box Tunnel. Photograph: Adam Scott/GWR

Golton said the tunnel had changed since it was completed in 1841. “Back then, it had 30m new bricks in the lining,” he said. ”It hadn’t had 130 years of steam locomotives through it to make it darker.”

It is possible that the sun used to glint off water in the tunnel – Network Rail continues to work hard to keep the tunnel dry. “Perhaps when it was first opened there was more water inside that the light could have been reflected off,” said Golton.

The supposed link with Brunel’s birthday was first reported by the Devizes Gazette in 1842. The Daily Telegraph followed the story up some time later but, until now, as far as GWR knows, there have been no photographs of the supposed phenomenon.

Golton said the idea of building such a long tunnel between Bath and Chippenham was hugely controversial and was described during a debate in parliament as “dangerous, extraordinary, monstrous and impractical”. Railway pioneer George Stephenson said passengers would be terrified.

The project was over budget and behind time. At the height of construction, 4,000 people were working on the tunnel and the engineers were getting through a tonne of candle wax and a tonne of explosives every week. One hundred people died during its construction.

It is not a surprise to Golton that Brunel might have added a mischievous detail to his astounding design. “There are lots of good reasons why Brunel might have wanted to provide a riposte to his critics by not only completing the structure but putting a special architectural signature into the job,” he said.

There will be no more practical chances to test the theory for the foreseeable future. The line is now open again and, from the autumn, new Intercity Express trains will begin operating on the line.