As the Seikan Tunnel — a rail link between Honshu and Hokkaido — marked the 30th anniversary of its opening on Tuesday, better coexistence between shinkansen services and freight traffic remains a key challenge.

At 53.85 km, the world’s longest undersea tunnel connects the northern tip of Honshu with Hokkaido. The tunnel, 23.3 km of which runs undersea, has been a major transport artery since its opening, as it is little affected by high winds and heavy snow.

The tunnel was the world’s longest railway tunnel until the opening of the Gotthard Base Tunnel in Switzerland in December 2016.

According to data compiled by JR Hokkaido Co., it took nearly 24 years and cost about ¥690 billion ($6.47 billion) to complete the tunnel’s construction, with some 14 million workers involved.

Construction of the tunnel, which runs 240 meters below the sea surface at its deepest point, was extremely difficult, with workers facing numerous problems including frequent landslides and flooding seawater. Thirty-four workers lost their lives.

The project got rolling amid a public outcry calling for a safer way to cross the Tsugaru Strait after a powerful typhoon sank the Toya Maru and other boats and killed over 1,100 people in September 1954.

The now-defunct Japan Railway Construction Public Corp. began boring in 1964.

In December 2017, the tunnel was selected as one of 20 symbols to represent the country’s technology and cultural heritage by the International Council on Monuments and Sites’ national committee in Japan. ICOMOS is a UNESCO advisory panel.

Today, the tunnel has grown busier, and officials are seeking a balance between passenger and freight trains. Freight trains going through the tunnel have been mostly transporting agricultural produce from Hokkaido to other parts of the country and bringing books and processed goods north.

About 50 freight trains and 30 shinkansen pass through the tunnel daily, with some 2.47 million tons of cargo, including 60 percent of the onions and 40 percent of the potatoes grown in Hokkaido, being distributed through the tunnel in fiscal 2015.

“Japan’s logistics have improved significantly thanks to the Seikan Tunnel. We will make efforts so that freight trains will continue playing a significant (logistical) role,” said Shuji Tamura, president of Japan Freight Railway Co.

Since the start of Hokkaido Shinkansen Line services between Shin-Aomori and Shin-Hakodate-Hokuto stations in March 2016, the companies have been working to find the best way for conventional freight trains and the high-speed bullet trains to share the tunnel.

Freight trains heading for Honshu must stop at Goryokaku Station in Hakodate, Hokkaido, to switch from diesel-powered locomotives to electric locomotives capable of running under a separate voltage system, because they share the rails with the bullet trains in the tunnel and its vicinity over an 82-km stretch.

In that section, trains run on dual-gauge tracks, with freight trains and bullet trains occasionally passing each other. Freight train delays are likely to affect the shinkansen schedule as well. “I operate with caution in order not to cause delays or other trouble,” a freight train driver said.

Another headache related to sharing the tunnel is a shinkansen slow-down. The Hayabusa shinkansen series, the fastest type of train in the country with a top speed of 320 kph, has to slow down to 140 kph in the tunnel to prevent freight train loads from falling — a risk caused by wind pressure created when the trains cross paths.

The speed limit in the tunnel may be raised to 160 kph, possibly in spring 2019, after test runs conducted by JR Hokkaido. That would shorten the travel time between Tokyo and Shin-Hakodate-Hokuto stations, the gateway to Hokkaido and the current terminus of the shinkansen line, by only three minutes from the current four hours and two minutes.

The Ministry of Land, Infrastructure, Transport and Tourism’s council for transport policy has considered a plan to adjust freight train timetables so that bullet trains and freight trains don’t pass each other in the tunnel. But doing so may restrict the amount of goods distributed throughout the country.

“As long as there’s demand (for rail freight), it’s difficult to reduce the frequency of service,” a Japan Freight Railway official said.

Given that the Hokkaido Shinkansen Line is set to be extended to Sapporo by the spring of 2031, the significance of the Seikan Tunnel is expected to increase, and railways will need to perform a balancing act in seeking faster passenger services and securing stable freight operations.