Pranksters caused thousands of pounds worth of damage by bricking up the door of an underground train, according to police.
Police in Germany said the perpetrators covered the entrance to a carriage on a Hamburg train using eight rows of breeze blocks.
The blocks were stuck together using expanding foam in the prank, believed to have been carried out on Tuesday.
Police are now reviewing CCTV footage from the train to try to determine who was responsible for the handywork.
The vandals are thought to have completed the job at a train depot above ground.
The passenger train, on Hamburg's S11 line, is thought to have passed through as many as 12 stations before staff noticed the blockade.
German police warned against "copycat acts" after the incident, which they said could lead to a prison sentence of up to five years.
