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For the opening of the system in 1940, the Seattle Transit System purchased trolleybuses from two different manufacturers. The Twin Coach Company provided 135 vehicles, while the J. G. Brill Company provided 100. All but the first Brill vehicles were built, under license, by Pacific Car and Foundry (PCF), in the nearby city of Renton, and are commonly referred to as PCF-Brill vehicles or simply Brills. Strong ridership growth during World War II, when the city's economy was booming, brought a need for additional vehicles. Forty-two more Twin Coach trolleybuses were bought in 1942–1943, followed by 30 Pullman-Standard vehicles in 1944. These were the last new trolleybuses acquired until the late 1970s. The fleet's size peaked at that time, at 307, making Seattle's trolleybus fleet the 12th-largest in the United States.