In the Dallas–Fort Worth area, three public transit agencies provide rail service: Dallas Area Rapid Transit (DART), Denton County Transportation Authority (DCTA), and Trinity Metro. DART began operating the first light rail system in the Southwest United States in 1996. The Trinity Railway Express (TRE) commuter rail service, which connects Fort Worth and Dallas, is provided by Trinity Metro and DART. Trinity Metro also operates the TEXRail commuter rail line, connecting downtown Fort Worth and Northeast Tarrant County to DFW Airport. The A-train commuter rail line, operated by DCTA, acts as an extension of the DART Green line into Denton County. In the Austin area, Capital Metropolitan Transportation Authority operates a commuter rail service known as Capital MetroRail to the northwestern suburbs. The Metropolitan Transit Authority of Harris County, Texas (METRO) operates light rail lines in the Houston area.