An improperly balanced wheel-load was the probable key cause of the subway train derailment that killed five passengers and injured 60 others on Tokyo’s Hibiya Line in March, a Transport Ministry expert panel said in its interim report released Tuesday.

Later the same day, Kiyoshi Terashima, head of Hibiya Line operator Teito Rapid Transit Authority, said he had tendered his resignation to Transport Minister Toshihiro Nikai to take responsibility for the accident.

“Since the accident, I have thought of eventually taking responsibility,” Terashima said at a news conference held at the ministry.

Terashima also said the railway operator will quickly compile accident prevention measures based on the report.

The accident took place during the morning rush hour on March 8, when the rear car of a subway train derailed and sideswiped a packed commuter train running in the opposition direction on a parallel track. It occurred about 100 meters northeast of Nakameguro Station in Tokyo’s Meguro Ward.

It was the first fatal accident Teito has experienced in the nearly 70 it has been operating.

The expert panel, headed by Tokyo University Professor Emeritus Masakazu Iguchi, concluded that the accident was probably caused by a combination of factors, including wheel-load imbalance, increased friction on the track, stiffness of the air suspension system, and surface area of the rail.

The panel, which conducted repeated simulations and on-the-spot experiments, specifically attributed the accident to a wheel-load imbalance that caused the wheels to climb on the track.

Teito did not regularly check or maintain wheel-load balances for subway cars in use.

Transport Ministry officials indicated that it may be necessary to introduce standards to check and maintain correct balances of train car body weight.

According to the ministry, no Japanese railway operator, except for Tokyu Corp., regularly checks wheel-load balances on its trains.

Tokyu began checking the wheel-load balance on its train cars after an imbalance caused a serious accident near Yokohama Station in 1986, the ministry said.

The panel will further examine to what extent each factor contributed to the derailment, and will try to compile its final report in the fall, according to ministry officials.

The panel measured wheel-load balances on all the subway cars of the same type as the one involved in the fatal derailment and found that wheel-load imbalances between right and left wheels on some cars reached nearly 30 percent.

The car believed to have caused the accident was badly damaged and it was impossible to measure its wheel-load balance afterward, but the panel concluded that it was highly unequal.