Glen Arm is an unincorporated community in Baltimore County, Maryland, United States. Glen Arm lies along what is now Glen Arm Road, which was once part of Old Harford Road, one of the early routes used for conveying agricultural products from parts of Harford and Baltimore Counties and southern Pennsylvania to the port of Baltimore. Glen Arm was also served until 1958 by the Maryland and Pennsylvania Railroad. In 1921, at the railroad's crossing of Glen Arm Road, what generally is recognized as the nation's first train-actuated railroad crossing signal was installed by the railroad's Superintendent of Signals Charles Adler, Jr. Adler later designed early traffic-actuated traffic lights for the City of Baltimore, and also invented the system of flashing warning lights used on aircraft.