In the early hours of Thursday, Tokyo Metro Co. discovered one of its trains had been vandalized overnight with graffiti while parked near Nakano Station, the subway operator said.

The incident was the third in the past seven days where Tokyo Metro subway trains had been vandalized, according to the firm.

The graffiti was discovered by a train conductor performing regular checks in preparation for the train’s first service of the day, at 5:08 a.m.

The Tozai Line train had been parked overnight in a rail yard after completing its last service at around 1 a.m., said Takahiro Yamaguchi, a Tokyo Metro spokesman.

The rail yard is in a staff-only area and is closed off from passengers.

Yamaguchi declined to give more details regarding how the vandals may have accessed the subway trains, citing security reasons.

The Tozai Line vandalism is the most recent of a slew of attacks in the past week, with a Hibiya Line train parked at Naka-Meguro Station vandalized Monday and another parked at Yoyogi-Uehara Station targeted Sunday.

Tokyo Metro has already lodged complaints with the police for the two earlier incidents. All the defaced subway trains ran along their regular routes before being pulled out of service at the nearest rail yards for cleaning.

Shigeo Kobayashi, a professor of urban architecture at Tokyo City University who has studied vandalism, explained that the motivation to vandalize trains is different from the motivation that underlies street graffiti.

The graffiti was presumably painted as a form of “self-promotion, self-expression, and also a boast,” he said. “It’s a way of showing off,” especially because vandalizing trains require calculation and preparation, he added.

The act of vandalizing a train is in itself thought to be a way of signaling status within the graffiti community, given that modern graffiti art started in the subways of New York.

Furthermore, he added, other countries are on high alert against terrorism and are strengthening their security.

Such factors may have made the Japanese subway system an attractive target for the vandals.