TOKYO - Japan's transport minister Keiichi Ishii has ordered the five Japan Railways (JR) companies that operate shinkansen bullet trains to conduct an urgent inspection of all 4,800 trains that are in service nationwide.

The move, announced Friday (Dec 15) after a Cabinet meeting, came after Japan suffered its first blip to its pristine bullet train safety track record earlier this week.

Following the discovery of a crack and an oil leak, a shinkansen train, Nozomi 34, was on Monday pulled from service, 3½ hours into its five-hour journey.

None of the 1,000 passengers was injured, preserving Japan's record of zero casualties since the shinkansen started running in 1964.

Mr Ishii stressed to reporters on Friday: "It is gravely important to ensure the absolute safety of the shinkansen."

But even as he noted that it was the first incident of its kind, he questioned why the train - run by JR West - was allowed to continue service for three hours after the first red flag, a burning smell, had been raised by the conductor.

JR companies have of late come under flak for a series of incidents.

On Wednesday, the shinkansen service Kodama 684 that plies the Nagoya-Tokyo route departed the terminal station without first opening its doors for the 200 waiting passengers to board. The train reversed 20m into the station, after the station master raised the alarm.

A spokesman for operator JR Central - also known as JR Tokai - said in a statement on Friday: "We must thoroughly counsel and educate all our staff to prevent a reoccurrence."

JR Central has also come under scrutiny as one of its employees is suspected to have leaked information during the tender process for a project to build emergency exits in Nagoya for a new maglev (magnetic levitation) line.

"Big four" construction company Obayashi is now being investigated for bid-rigging and collusion based on market-sensitive information allegedly provided by the JR Central official.

JR Central, too, manages the Nagoya station where the decision was made to pull Nozomi 34 from service at 5pm on Monday after the discovery of a crack and an oil leak.

The eastbound train had started its journey at 1.33pm from Hakata station in Fukuoka in the south-west, and was bound for the main Tokyo station.

In using the term "serious incident" for the first time, the Japan Transport Safety Board said the "structural anomaly" measuring more than 10cm could have led to a deadly high-speed derailment.

The board's investigators completed their official probe on Thursday, and work is now ongoing to dismantle the train at the Nagoya station so it can be safely moved to a nearby carriage base for repairs.

Given that the station has had to operate all week with one less platform, station services have generally been delayed by up to 10 minutes all week, JR Central said.

One of these was Kodama 684, which left the station at 8.31pm - two minutes later than scheduled - on Wednesday night, but without the waiting passengers on board.

While there are usually three train conductors on board, there had been only one conductor on the train, the Yomiuri daily reported.