The St. Charles corridor extends from northwest St. Louis County to St. Peters in St. Charles County, Missouri.  The 1991 Systems Analysis prioritized the St. Charles corridor as a Tier I priority.

In 1996, an alternatives analysis and environmental analysis evaluated potential alignments from the original MetroLink line to St. Peters in St. Charles County.  Build alternatives for light rail included alignments from Lambert Airport generally along I-70 or from North Hanley via Westport and Earth City to along I-70.  Also in 1996/1997, St. Charles County voters twice defeated a local sales tax proposition to for MetroLink expansion. Thus, due to the lack of any local funding match, no project advanced from the 1996 MIS/DEIS.