British Transport Police want to speak to the man pictured after faeces was found on a chair at a train station on the London Midland line.
A man was seen pulling down his trousers and sitting on a coffee shop chair at Rugby Train Station and faeces was later found in the same area by cleaning staff.
The incident happened at 10.45pm on August 28, police revealed today.
Police are hoping commuters who use the London Midland line, which runs from Euston and passes through several Dacroum areas including Kings Langley, Apsley, Hemel, Berkhamsted and Tring, may recognise the man.
Anyone with information can call BTP on 0800 40 50 40 or text 61016.