This is a southeast to northwest 14-mile branch line built around 1920 - 1930 as a "release valve" for the Missouri Pacific (now Union Pacific) east-west main line out of St. Louis, MO to Kansas City, MO. The main cuts almost right through the middle of St. Louis City and the outer suburbs (known collectively as St. Louis County), and there were probably more grade crossings than there is now, which are still quite a few.
In the early part of the century, trains on the main into St. Louis had to wait up to 6 hours before entering the city, blocking many of these at-grade crossings. The city leaders at the time therefore gathered and asked/requested the MP to build an additional line to route trains to the south and around St. Louis to relieve this rail congestion. It was decided that this "release-valve" line would begin in the town of Kirkwood, MO, about 7 miles to the west of St. Louis (thus the nickname "Kirkwood Cut-off"), and snake its way south around the city, where it would enter into the extreme south end of the city of St. Louis and into the neighborhood know as Carondolet, where it would hook up, by way of a wye, with the north-south line of the MP out of St. Louis. The line was finished around 1930, and was used as a freight train by-pass until the early 1990s.
Of note is the fact that this line never saw passenger rail traffic except once, when a troop train about 1943 was loaded at Jefferson Barracks, a former army base in south St. Louis, and headed northwest on the branch line to the east-west MP main and out west to the Pacific rim. By 1992, traffic on the line had diminished to only a few industries that were being served along the route, so the line was cut in the middle and roughly a 6 to 8 mile section was made into Grant's Trail. (Grant's Farm, a tourist attraction after General Grant who lived there for a time, is along a 1 mile section of the line/trail, hence the connection.) About 2.5 miles was then left to the north and about 3.5 miles to the south. The north end survived until about 1999, when the BNSF railroad cut the line in Kirkwood where it crossed the BNSF's east-west double main by a double diamond about half a mile after it leaves the UP main (the few industry sidings along this 2.5 mile section were no longer being served by that time). This two-mile section, with no connection on either end, remarkably stayed intact for almost another two years, until 2001 when the tracks were taken up and plans to extend the trail were made. The trail extension, however, has hit a snag due to financial dealings, but will probably be completed soon.
What is left of the line is a empty half-mile section to the north, sometime used to store MOW equipment, and the 3 mile section to the south, which is still in use to serve a recycling plant at the end of the line, which receives about 5 boxcars at a time.
Thanks to Brian Contestabile for contributing information about this route.