Police and Transport Ministry officials held predawn test runs Wednesday in their probe to determine the cause of a train derailment and subsequent collision near Nakameguro Station on the Hibiya Line in Tokyo that killed five passengers and injured nearly 40 on March 8.

Before starting the four-day series of test runs, railway workers reinstalled four 25-meter rail sections that had been removed and confiscated by police as evidence as part of their investigation into the crash.

Transport Ministry officials also used various parts of the coaches that were involved in the accident in a bid to re-create the derailment situation and ensuing deadly collision. In the crash, the lead truck of the rear car of one train derailed on a curve, causing the car to sideswipe a crowded oncoming commuter train on a parallel track.

The ministry will continue the test runs by varying the speeds, the load balance on the wheels and the rail lubrication.

Railway experts at the ministry and police have yet to detect any particular abnormalities on the cars and tracks, leading them to believe the derailment was the result of a combination of factors.

“(The investigation) has been quite tough because nothing in particular appeared to be wrong,” a ministry official said.

Data will be collected on such factors as wheel loads, sideways torque, air pressure in the undercarriages and car vibration.

An investigative panel under the ministry will input the data into computer simulation models to examine which factors would most likely cause a derailment, ministry officials said.

Cracks in bullet trains

Parts manufactured by Sumitomo Metal Industries Ltd. that are used in the undercarriages of bullet trains were found to have fine cracks in them, East Japan Railway Co. said Wednesday.

Two bullet trains were found to have 9-cm cracks in parts that support the main motor.

The cracks, even if left unattended, would not immediately affect the bullet trains’ operations, JR East claimed.

But the company has cautioned Sumitomo Metal Industries because the cracks appear to have been caused by incorrect welding, the railway operator said.

The Transport Ministry has ordered all railway operators across the country to check undercarriages made by Sumitomo Metal after similar cracks were found on a number of undercarriage parts made by the firm.