Even small delays in Japan's much-vaunted bullet trains are rare, and more unusual still are snakes on board holding up the speedy shinkansen services.

On Tuesday evening, a passenger of a train that arrived in Tokyo alerted security to a 40-centimeter serpent lurking on the train between Nagoya and Tokyo, resulting in a 17-minute hold-up as officials found and captured it.

It was unclear whether the cold-blooded commuter was venomous or how it ended up on the train, and there was no injury or panic among passengers, a spokesman for Central Japan Railway said.

Shinkansen customers can bring small dogs, cats and other animals, including pigeons on board — but not snakes.

"It's difficult to imagine wild snakes somehow climbing onto the train at one of the stations. We have rules against bringing snakes into the shinkansen," the spokesman said.

"But we don't check passengers' baggage," he said.

The train was originally scheduled to go on to Osaka, but the company decided to use a different train for the trip, causing a delay of about 17 minutes, he said.

Patrols by uniformed security guards onboard bullet trains were scaled up after a fatal stabbing in 2018 on a shinkansen that shocked normally ultra-safe Japan.

Additional security was added for the Summer Olympics in 2021 and Group of Seven meetings last year.

First launched in 1964, the shinkansen network has never suffered an accident resulting in any passenger fatalities or injuries, according to Japan Railways.

The trains can travel at 285 kilometers per hour, with an average delay of 0.2 minutes.