Much of the southern half of the Blue Line follows a former TRRA/Rock Island railroad right of way that ran from BNSF's junction with the Cuba and River Subdivisions (known as Southeastern junction) to what is now I-170 Business Center in St. John. The current stretch of line from Forest Park to Clayton Stations was known as the Forsyth branch until Rock Island abandoned it in 1931. The Terminal Railroad Association of St. Louis (TRRA) used the portion from Clayton Station onward, as well as the rest of the portion from St. John to Clayton until 1987 when the portion from Big Ben to Olive Boulevard was severed to the lack of customers on that stretch. This was known as TRRA's Central Belt. The rest of the track was removed in 2002 just prior to construction on the Blue Line. All that remains of the original Central Belt is the portion north from Rock Island Junction, near Overland, MO (where the Central Belt crossed the junction between TRRA's West Belt and the Missouri Central (now Central Midland) Railway, to just a fifth of a mile past Page Avenue where less than 10 industries still use TRRA.
The Blue Line is part of the Cross County Corridor. These following extensions will include up to 46-mile (74 km) of track, but in order to plan, design, and construct these separate extensions, they will need anticipated local and federal funding.
According to the East-West Gateway Council of Government, it has been identified as the next priority. The Daniel Boone Corridor will extend from the Shrewsbury I-44 Branch (Blue Line) to West St. Louis County, initially terminating near Westport Plaza in Maryland Heights. The 8–10-mile (13–16 km) extension will add up to six stations between Clayton and Maryland Heights in the I-170-Page Corridor serving Ladue, Olivette, and Overland. A recent study shows the alignment might branch off northwest to St. Charles County.
Phase II - Westport to Chesterfield
This 12-mile (19 km) extension might become a Phase II of the Daniel Boone Corridor. The alignment crosses west over the Interstate-270/255 Ring in Maryland Heights. It will run along with the Page Ave Extension in North County until it crosses off near Creve Coeur County Park. It then will head westward to the Spirit of St. Louis Airport in the Chesterfield Valley. This alignment has not been given serious consideration because of its perceived low ridership potential.
This 12-mile (19 km) extension will cut off from the Daniel Boone Corridor just north of Clayton in Olivette and Overland to continue north along I-170 to Florissant, Ferguson, Hazelwood, and Berkeley in North County. Most of the line will use the old Rock Island Railroad right-of-way, right alongside I-170. It will continue 3 miles out of the I-170/I-270 interchange in Hazelwood.
This 12-mile (19 km) extension will begin from the Shrewsbury-Landsdowne/I-44 station, following Watson Road in Shrewsbury. The corridor follows a use right-of-way further south to Mehlville, Oakville, Affton, and Butler Hill in South County. The alignment will serve along with the Grant Bike Trail. This extension was deferred in 2005 due to lack of funding.