The A Line is a Regional Transportation District (RTD) commuter rail line serving Denver and Aurora, Colorado, operating between downtown Denver and Denver International Airport (DIA). During planning and construction, it was also known as the East Rail Line, but most locals refer to it as the A Line. Despite its former title, the line does not serve the campuses of the University of Colorado.

The Regional Transportation District, more commonly referred to as RTD, is the regional agency operating public transit services in eight out of the twelve counties in the Denver-Aurora-Boulder Combined Statistical Area in the U.S. state of Colorado. It operates over a 2,342-square-mile (6,070 km2) area, serving 3.08 million people. RTD was organized in 1969 and is governed by a 15-member, publicly elected Board of Directors. Directors are elected to a four-year term and represent a specific district of about 180,000 constituents.