Transit Police have released photos of a man they say "engaged in an a lewd act" on a Red Line train between Alewife and Davis around 8:50 p.m. on Dec. 30.
He's described as white, 25 to 35, clean shaven and 5'10" to 6' with a medium build.
If you know a guy who can't keep it zipped, contact Transit Police at 617-222-1050, or send an anonymous tip to 873873.