Posting a letter in 1927 was the equivalent of using Whatsapp today. It was instant messaging, early 20th century style.

Islington Gazette: Post Office underground railway: car depot and workshop at Mount Pleasant. Picture: Royal MailPost Office underground railway: car depot and workshop at Mount Pleasant. Picture: Royal Mail (Image: Archant)

At its peak, four million letters a day would go through Mount Pleasant Mail Centre’s underground rail service.

Later branded “Mail Rail”, the letters would be plonked onto a driverless train to Paddington station.

It opened in 1927, closing in 2003. But the heritage is not lost. On Friday, the Postal Museum, next to Mount Pleasant, opened to the public in Phoenix Place.

And on September 4, it will be followed by the re-opening of Mail Rail across the road. Visitors will be able to experience a subterranean world previously closed to the public.

Islington Gazette: A Mail Rail carriage. Picture: James MorrisA Mail Rail carriage. Picture: James Morris (Image: Archant)

Today, the Gazette was lucky enough to hop on a train for a preview tour ride. Many of the platforms remain unchanged, down to the musty smell and a Winmau dartboard on the wall, which engineers would use to pass time in between carriage stops.

For Tim Ellison, deputy director of the Postal Museum, it’s a celebration of a “massive piece of industrial history”.

He said: “The Post Office has a real tradition of innovation, and this was part of that.

“At the time of construction, most materials were delivered via horse-drawn carriage in London, causing huge congestion in the streets. This underground system was designed to solve that.

Islington Gazette: Tim Ellison, deputy director of the Postal Museum, on the underground Mail Rail tracks. Picture: James MorrisTim Ellison, deputy director of the Postal Museum, on the underground Mail Rail tracks. Picture: James Morris (Image: Archant)

“It massively sped up the process from A to B. Letters would be loaded onto trains here, transported to Paddington and distributed from there.

“Mail was taken incredibly seriously at the time. If a letter was late, whoever was responsible had to explain why. People used to set their watches by the arrival of mail coaches.

“Here, there were four million letters every day, with trains running every seven minutes and for 22 hours a day. It gives you an idea of the importance of mail – people used it to arrange meetings like we would with Whatsapp today.”

After 76 years, Mail Rail was no longer needed. But Tim added: “It absolutely did its job. It was incredible innovation, but the world around it changed by 2003. Of course, not as many letters were being sent.”

Islington Gazette: Post office underground railway: workers load the train. Picture: Royal MailPost office underground railway: workers load the train. Picture: Royal Mail (Image: Archant)

Over the years, ideas for the vacant Mail Rail site included a mushroom farm and a cycle superhighway. But Tim, who first started working on the museum project in 2009, is glad its legacy has been preserved: “We have stories people haven’t heard before. It’s an industrial legacy.

“This underground section would be under threat otherwise. It doesn’t have listed status, and if people wanted to build above the ground, it could damage the infrastructure. By opening up I feel we are preserving it for many years to come.”

Ray Middlesworth, 59, lives in nearby Clerkenwell Road. He joined the Royal Mail as an apprentice in 1974 and started working on Mail Rail as an engineer in 1987. When it closed in 2003, he was the man who shut the power off.

He and three other colleagues stayed on in the intervening years. As a scheme built with public money, Royal Mail had a duty to maintain the tunnels, meaning Ray stayed underground. He will now work for the museum as an advisor.

Islington Gazette: Engineer Ray Middlesworth started work on the Mail Rail in 1987. Picture: James MorrisEngineer Ray Middlesworth started work on the Mail Rail in 1987. Picture: James Morris (Image: Archant)

“When we shut the power off in 2003, it was a moment in history,” he said. “It was great working down here.

“One of the best moments was when scenes from Hudson Hawk [starring Bruce Willis] were shot here. We manually controlled the trains. It was a high pressure job, as we had to make sure world famous film stars got home safely!

“We were still happy working down here after it closed. The only thing was, there used to be hundreds of us down here, so it took a while getting used to it being so much quieter.”

Visit postalmuseum.org for more.