"Ghost" marks are appearing at Elizabeth line stations in the form of grey outlines of seated commuters.
Photos from Tottenham Court Road, on Transport for London's (TfL) newest line, show grey outlines of people.
The marks can be seen at Liverpool Street, with the station's white walls apparently stained by commuters' coats and heads.
The Elizabeth Line, named after the late Queen Elizabeth II, opened on May 24 2022 - 13 years after construction on the line began in 2009.
The 41-stop route now hosts 204.296 million passenger journeys per year, costing the Government around £18.9 billion.
Transport for London, which operates the line, says that it maintains a "rigorous" cleaning programme at all stations to ensure they are safe for commuters.
A spokesperson for the TfL said: "We are exploring new ways to clean the walls behind seating at some of our Elizabeth line stations to try to remove any marks and protect the walls in future."
The Elizabeth line connects Abbey Wood and Shenfield in the east to Reading and Heathrow in the west.
It recorded 738,000 journeys on September 28 2023, the highest the new line has seen. That week saw the highest total with 4.5 million journeys made.
Howard Smith, Elizabeth line director, said at the time: “The Elizabeth line has rightly and quickly become one of the most popular railways in the country. Recently we have seen impressive passenger numbers on the Elizabeth line and we were pleased to see that on September 28 2023 we recorded the highest daily number of Elizabeth line journeys since opening in May 2022 with 738,000 journeys."
The Elizabeth has faced chalenges, with 9.1% of services not running in the four weeks to August 19.
Smith apologised for a “difficult period which included disruption for our customers”, continuing: “We continue to work with all parties involved in the Elizabeth line to provide a safe and reliable railway. This includes our partners, such as Network Rail, and a programme of upgrades delivered by the train manufacturer, Alstom, will further improve the reliability of our fleet of trains.”