"This was a relatively small but diverse labor organization. In 1944, there were 125,000 members in 435 locals of this union that was headquartered in Detroit, Michigan. By that time, the union included motormen, conductors, guards, brakemen, trolleymen, bus and coach operators, garage men and all men operating cars and trams, all gatemen, watchmen and Wardens, all employees of the car houses and pit department, all collectors, janitors, watchmen, yard crews, elevator men, porters, clerks, trackmen and laborers. One has to wonder why there are not more mugs depicting this trade with so many jobs included within the union. In addition, individuals who held minor position such as starter, dispatcher, inspector, timekeeper, streetmen or barn shop and track foremen, "working with their fellow workers and not having the power of discipline in the way of hiring, suspending or discharging employees," (that is, non-supervisors) could join the union.
The union was organized in 1892 in Indianapolis, Indiana and initially represented those working with horse drawn and electric trolley and interurban rail lines. These individuals primarily moved passengers, not freight, and were not directly connected with the interstate railroads that crossed the country during this period of industrial growth. The mug depicts an electric street car with the letters "AA of S & ERE of A" above it."
From The Historical Society of Pennsylvania:
The Amalgamated Association of Street and Electric Railway Employees of America (Amalgamated) formed in 1892. Arguing for increased benefits and compensation, Amalgamated organized successful, but occasionally violent, strikes of Philadelphia Rapid Transit Company workers in 1909 and 1910. The union continues today as the Amalgamated Transit Union (ATU).