The Cross County extension is the third and latest MetroLink corridor under construction.  Anticipated for completion in 2006, Cross-County travels from the redesigned Forest Park station on the original line via Clayton to a new terminal station at Shrewsbury-Lansdowne I-44 via Clayton. This new eight-mile extension adds nine new stations and is the first branch extension. 

The 1991 Systems Analysis prioritized the Cross County corridor as a Tier I priority for consideration of a light rail extension. As part of a 1997 multi-modal study, the Cross County Major Transportation Investment Analysis (MTIA), East-West Gateway identified transportation problems and potential solutions were identified in in the north-south and east-west areas surrounding central St. Louis County known as the Cross County Corridor. Out of the 1997 MTIA came a number of improvements being selected for further consideration including the proposed extension of light-rail to Clayton off the original line, as well as conceptual connections to Mehlville in south St. Louis County and Florissant in north St. Louis County.  Potential extensions beyond the Cross-County MetroLink line under construction to I-44 are now known as Metro South and Metro North.  After two more years of detailed planning activities, in 1999 the East-West Gateway Board selected the preferred alignment and conceptual design for the Cross County extension.

The Board also agreed to fund the construction of the Cross County line with local funds.  This funding relies on bonds backed by a local proposition passed in 1997, for a  Prop M, a qquarter-cent sales tax levy in the City of St. Louis and St. Louis County.

The "Selected Alignment" map to the right gives an overview of the selected alignment, for which conceptual design completed in 1999 was at a 10% engineering level. The most noticeable change from the 1997 Strategic Alignment Analysis to the 1999 Conceptual Design was downtown Clayton, as the alignment was now planned to run along Forest Park Parkway instead of Carondelet Avenue through Clayton.  In 1999, upon Board approval of the conceptual design, East-West Gateway handed off the project to Metro (Bi-State Development Agency), the St. Louis region's major transit authority, for implementation. The new Cross County extension is scheduled for completion in 2006.