A handout photo released by the German Federal Police Bundespolizei on April 29, 2015 shows the door of an S-Bahn subway train carriage, which has been bricked up in Hamburg, Germany. (AFP)
German police are used to chasing graffiti artists defacing urban passenger trains but were baffled to find someone had bricked up a carriage door.
One or more unknown pranksters stacked up eight rows of cement blocks and glued them together with spray foam to entirely close off the train car door, police said today.
They estimated the property damage at tens of thousands of euros from the act of vandalism in the northern city of Hamburg, where the perpetrators struck some time Tuesday.
Federal police said they would review security camera footage from the train to identify the vandals who may have struck at a depot of the above-ground S-Bahn rail service.
The train had passed through more than a dozen stations along Hamburg's inner city S11 line before railway staff noticed the bizarre prank.
Police issued an "urgent warning against copycat acts", warning the subversive bricklayers could face fines and up to five years' jail.