The line from Downtown to SEATAC is technically "Light Rail." Portland, OR also has both light rail and streetcars for example. New Orleans has streetcars (aka, trolleys) whereas Sacramento has light rail. But these two concepts blur such as the Shaker Heights line in Cleveland and suburbs (the line looks like light rail in the city and streetcars in the suburbs). Then you have rapid transit (often called subways or Els) such as the LA Red Line, the New York Subways. Heavy Rail is commonly called commuter rail (CalTrain, Long Island RR), but then you have BART which is a cross between rapid transit (in San Francisco and Oakland) and commuter rail in the suburbs. In the progression from streetcar to commuter rail the vehicles get heavier with more capacity and speed with stops farther apart (in general).

Modern trolley systems (Seattle, Tacoma, and Portland) are not trolley's because they collect electricity via a pantograph and not a trolley pole. The origins of the word trolley is unclear. Originally streetcars picked up both the hot and the ground (DC electricity) from two overhead wires. A little cart on wheels rolled along the overhead wires to pick up the current and was connected to the trolley car via an extension cord which also pulled the car along. I have heard the word trolley came from trolling like you do for fish (i.e., trolling the little cart). Also I have heard the little cart was likened to the British tea trolley, a wheeled cart for serving tea.