Tokyo-based subway operator Teito Rapid Transit Authority said Thursday that it will check and correct wheel-loads on all its 2,431 train carriages by the end of this year, based on a report by a Transport Ministry study panel.
The expert panel concluded in the report, released Tuesday, that an imbalance in the wheel-load was probably the main cause of the derailment, which occurred near Nakameguro Station on the Hibiya Line and killed five commuters and injured 60 others.
After the accident, the panel examined all the cars on the Hibiya Line and determined that the wheel-load imbalance on some carriages reached as much as 30 percent.
By mid-June, the subway operator had already corrected wheel-load imbalances on all cars on the Hibiya Line to less than 15 percent, the firm said.
Teito has set up wheel-load measuring machines at six depots and will regularly check the wheel-loads of all train carriages, the subway operator said.
Teito said it will also continue to study other probable causes identified in the report, such as friction conditions, stiffness of wheel suspension and the surface of the railway track.
Currently, train carriage manufacturers' verification standards for wheel-load imbalance is up to 20 percent.
Teito officials said they will request that carriage makers introduce tighter quality control standards.