BEIJING - Photos of grimy high-speed trains, seen as evidence of how bad air pollution has become, have gone viral in China.

The white trains turned grey after travelling in the north of China.

They were published by a photographer from The Economic Daily newspaper , who took the train from Shanghai to Beijing, the South China Morning post said.

More photos of dirty trains were shared by netizens on social media later.

PeopleRailway.com, a government website, posted that the trains had to be cleaned twice on the same day.

Beijing issued its highest red fog alert on Jan 1 and Jan 4.

China's weather bureau warned of visibility of less than 50 metres in some areas, leading many airports to cancel flights.