Police on Saturday warned Chicago Transit Authority riders of a series of robberies on Pink Line trains this month.

Two men, believed to be between the ages of 20 and 30, walk up to commuters, grab their cell phones and then run off when the train stops at a station, according to a police alert.

Police said the robberies happened at 10:40 p.m. Dec. 10th at the Kostner stop; 6:50 p.m. Dec. 15th at the Central Park stop; 6:04 p.m. Dec. 16th at the Pulaski stop; and 11:20 a.m. Dec. 18th at the Kedzie stop.

While descriptions of the suspects were sketchy, police were told one of the robbers was so tall that his head almost touched the top of the train door, the alert said.