Authorities have no idea why the straps are being taken but at least 400 have been removed from trains since November according to the Kyodo News Service.

The straps have little value and are difficult to remove, leading investigators to suspect the practice is pure vandalism – something that is rare in Japan.

A railway spokesman said: “I have no idea what this person is going to use them for, but it causes trouble for our customers so I hope this person will stop doing this.”

The Den-en-Toshi line has lost 182 straps, leaving commuters in awkward and uncomfortable positions on rush hour trains.

Passengers also risk injury without them which is why they are produced to high standards their manufacturer told Kyodo.