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In 2004 the Metropolitan Transit Authority of Houston/Harris County opened the 7.5-mile Metrorail Red Line, a light-rail transit line that traverses downtown on Main Street. Hellmuth, Obata and Kassabaum (HOK) designed Metrorail's transit station prototype, PGAL Architects designed the Preston Avenue and Main Street Square stations, and HOK the Bell and Downtown Transit Center stations. M2L Associates were the landscape architects. The Houston Arts Alliance helped select artists to carry out public art installations at each station; Tim Glover was in charge of the Preston and Main Street Square stations. In addition to its public transit role, the Metrorail project dramatically improved the appearance and spatial feel of Main Street. By filling up a wide expanse of asphalt, it made the street into a linear urban room, especially perceptible in the Main Street–Market Square Historic District, where the rehabilitation of historic buildings has contributed to the construction of a much more amenable public environment. The Houston Downtown Management District coordinated with Metro and the City of Houston in making these dramatically positive changes to Main Street.