Over the past few months unidentified suspects have been taping up any holes – like money slots – in train tickets machines before filling them with gas and attempting to blow them up.

One machine in Karben, near Frankfurt, was shut after police had to defuse a gas-filled ticket machine.

The official warning, coming from the Hessian state office for criminal investigation and the police, said the volatile gas mixture didn’t always ignite, but there was still a danger even after the culprits had given up.

Passengers should avoid ticket machines that have been taped up, and let the police know immediately, they said.

A spokesman for Germany’s main train operator Deutsche Bahn (DB) admitted to the paper that “although suspects wouldn’t be able to get away with much money because of how regularly the machines are emptied, it does seem that this is criminal activity.”

Deutsche Bahn has stepped up its safety policy since an initial explosion and is keeping a closer eye on ticket machines, the spokesman added.