TOKYO -- Japan's bullet train planners will be experimenting, with stopwatch in hand, ways to increase speed for a planned shinkansen connection between Tokyo and the northern city of Sapporo.

Their goal is to complete the trip in four and a half hours, which they believe is the maximum to convince travelers to switch over from airplanes. But to do that, they have to tackle a bottleneck: potatoes and onions.

The biggest barrier to faster speed is the Seikan Tunnel, the 53 km underwater railway tunnel that connects Hokkaido with the main Japanese island of Honshu.

The three-track tunnel is used jointly by Hokkaido Railway, the bullet train operator otherwise known as JR Hokkaido, and Japan Freight Railway, which carries cargo on non-high-speed trains.

It is the potatoes, onions and other produce that Japan Freight carries through the Seikan Tunnel that are at issue. Due to the risk of cargo falling off a freight train as a shinkansen whooshes past, the bullet trains are restricted to 160 kph in the tunnel, far below their maximum speed of over 300 kph.

One possible answer is for these bullet trains to carry cargo themselves. JR Hokkaido has partnered with delivery company Sagawa Express to test transporting seafood on bullet trains alongside passengers. The trials will start this year at the earliest.

Before trains enter the tunnel, JR Hokkaido and Sagawa plan to load squid, fish and other seafood caught near Hakodate onto a shinkansen heading toward Aomori, Honshu's northernmost prefecture. The two companies anticipate strong demand for the freshest seafood, enough to command high transport fees.

Fishermen land a catch of squid at a wholesale market in Hakodate, Hokkaido. © Reuters

The exact products to be shipped will be determined later. Potatoes and onions will not be delivered this way, because no producers of those crops would pay the high transport costs.

The downside of this plan is that large volumes of cargo cannot be transported. The approach, therefore, will be combined with other options being examined, including establishing time slots for bullet trains to travel at top speed through the tunnel. Freight also could be transported by ferry instead of rail.

A trip from Tokyo to Shin-Hakodate-Hokuto Station at the southern tip of Hokkaido currently takes roughly four hours. With a 200 km extension to Sapporo, the capital of Hokkaido, expected to be complete around 2031, the challenge is to realize the full trip under four and a half hours.

Resolving the bottleneck at Seikan Tunnel is crucial for the shinkansen to compete with airlines.

The problem surrounding joint use of Seikan Tunnel cannot be solved solely by the two rail companies, JR Hokkaido President Osamu Shimada said. "It cannot be resolved through a single approach," he added.

Sagawa, the main unit of SG Holdings, apparently pitched the pilot project to JR Hokkaido. The two sides have achieved positive results using non-shinkansen rail lines.

JR Hokkaido, Sagawa and another company started transporting freight and passengers together on a trial basis in April between the city of Wakkanai and the town of Horonobe, both in Hokkaido. The partners drew praise for their efforts recently from Kazuyoshi Akaba, Japan's transport minister.

Many stakeholders depend on the success of the shinkansen project. For fiscal 2018, JR Hokkaido turned in a consolidated operating loss of 41.8 billion yen ($384 million). The Hokkaido shinkansen generated a 9.5 billion yen loss that year.