A staff member in clown costumes waves to passengers in Chicago, the United States, on Dec. 18, 2019. To celebrate the forthcoming Christmas holiday season, the Chicago Transit Authority (CTA) has introduced Christmas Train services. (Photo by Joel Lerner/Xinhua)

A train painted with Christmas patterns is seen in Chicago, the United States, on Dec. 18, 2019. To celebrate the forthcoming Christmas holiday season, the Chicago Transit Authority (CTA) has introduced Christmas Train services. (Photo by Joel Lerner/Xinhua)

A staff member in Santa Claus costumes waves to passengers in Chicago, the United States, on Dec. 18, 2019. To celebrate the forthcoming Christmas holiday season, the Chicago Transit Authority (CTA) has introduced Christmas Train services. (Photo by Joel Lerner/Xinhua)

A boy peers inside a Christmas Train in Chicago, the United States, on Dec. 18, 2019. To celebrate the forthcoming Christmas holiday season, the Chicago Transit Authority (CTA) has introduced Christmas Train services. (Photo by Joel Lerner/Xinhua)