The origins of the Light Rail ultimately lie in a transit plan drawn up for the Baltimore area in 1966 that envisioned six rapid transit lines radiating out from the city center. By 1983, only a single line had been built: the "Northwest" line, which became the current Baltimore Metro Subway. However, much of the plan's "North" and "South" lines ran along right-of-way that was once used by interurban streetcar and commuter rail routes—the Northern Central Railway, Washington, Baltimore and Annapolis Electric Railway, and Baltimore and Annapolis Railroad—that still remained available for transit development.
Beginning in the late 1980s, Governor William Donald Schaefer (a former mayor of Baltimore) pushed for building a transit line along this corridor, motivated in part by a desire to establish a rail transit link to the new downtown baseball park being built at Camden Yards for the Baltimore Orioles. The Light Rail line was built relatively quickly and cheaply, and without money from the U.S. federal government, a rarity in late 20th century U.S. transit projects. The initial system was a single 22.5 mile (36 km) line, all at grade except for a bridge over the Middle Branch of the Patapsco River just south of downtown Baltimore. The line ran from Timonium in Baltimore County in the north to Glen Burnie in Anne Arundel County in the south. The Light Rail began service in April 1992, the same month that the Orioles began play at Camden Yards.
Three extensions to the system were added in 1997. In September of that year, the line was extended north 4.5 miles (7 km) to Hunt Valley, adding five stations that served a major business park and a mall. In December, two short but important spurs were added to the system: one a 0.34 mile (550 m) spur in Baltimore that provided a link to Penn Station, a transit hub also served by MARC and Amtrak trains, and the other a 2.7 mile (4.3 km) link that brought trains to the terminal of BWI Airport.
In 1998, the Hamburg Street station opened as an infill station between the existing Westport and Camden Yards stations. Adjacent to M&T Bank Stadium, it was initially only open during Ravensgames and other major stadium events; however, in 2005, it became a full-time stop.
To save money, much of the system was built as single-track. While this allowed the Light Rail to be built and opened quickly, it made it difficult to build flexibility into the system: much of the line was restricted to 17-minute headways, with no way to reduce headways during peak hours. Federal money was acquired to make the vast majority of the system double-tracked; much of the line south of downtown Baltimore was shut down in 2004 and north of downtown shut down in 2005 in order to complete this project. Much of the northern section reopened in December 2005; the rest opened in February 2006.