The wheels of the last car of a subway train and the track it was running on may have been improperly shaved and polished, possibly contributing to Wednesday's deadly accident, informed sources said Thursday.

The ministry's investigative team has discovered that the Hibiya Line track around the site of the accident in Tokyo was shaved about three months ago, while the wheels of the car were polished some five months ago, the sources said.

Insufficient polishing work or too much shaving could have contributed to the accident, in which the car derailed and plowed into several cars of an oncoming train, killing four people and injuring 35 others, they said.

They also found that the flange -- the protrusion on the inner side of a wheel that secures it on the track -- on the right wheel of the car that derailed showed traces of having run over the track for about 5 meters before actually jumping the track, the sources said.

According to officials of Teito Rapid Transit Authority, which operates the Hibiya Line, workers shaved off 0.2 mm to 0.3 mm of the surface of the track around the accident site Dec. 18 last year using an automatic shaving machine.

The wheels of the derailed car underwent a similar operation on Oct. 5, 1999, when the diameter of the wheel was reduced from 839.2 mm to 834.4 mm and the length of the flange was shaved down from 30.1 mm to 27.9 mm.

Periodic shaving operations are conducted to remove scratches and dents that develop on the surface of tracks and wheels, according to ministry officials.

Metropolitan Police Department officials said earlier they had found that the derailed car, believed to have plowed into the fifth and sixth cars of an oncoming train, had also sideswiped the fourth car.

The MPD is conducting a joint investigation with the ministry's team.

The collision occurred at 9:01 a.m. Wednesday on a stretch of aboveground tracks about 150 meters northeast of Nakameguro Station in Meguro Ward.

The wrecked trains were brought Thursday morning to Teito's train maintenance station in Tokyo's Arakawa Ward. Police have already confiscated rails and railroad sleepers from the accident site.

Police said they are also questioning the trains' operators and subway officials on suspicion of professional negligence resulting in death and bodily injury. The six-member Transport Ministry team also questioned subway officials.

All services on the Hibiya Line were back to normal Thursday, with the day's first train departing on schedule at 5 a.m., some 20 hours after the accident, subway officials said.

Teito employees worked through the night installing a guard at the accident site to prevent another derailment and checked the undercarriages of 336 subway trains of the same type as the derailed train.

Yasuyo Maki, a 37-year-old employee of the Minami-Nippon Shimbun's Tokyo Bureau, Shinsuke Tomihisa, 17, a second-year student at Tokyo's Azabu High School, Tomomi Yamazaki, a 29-year-old company employee, and Shinya Fujii, a 33-year-old Patent Office employee, were killed in the accident, police said.

According to police, the tail car of the Teito-operated southbound eight-car train, which was about to arrive at Nakameguro Station, derailed on a curved part of the track about 50 meters south of a subway tunnel exit.

It slammed into the fifth and sixth cars of the eight-car train on the opposite track, police said earlier. The concrete sleepers on the southbound track were damaged more than 82 meters from the point of the derailment, they said.

The train, which had just left Nakameguro Station bound for Takenotsuka, northern Tokyo, was operated by Tobu Railway Co., whose trains are allowed on Teito tracks.

"We have not been able to find material that could immediately shed light on the exact cause of the accident," said Administrative Vice Transport Minister Hisashi Umezaki. "I think it will take some time before we know all the facts."