St. Louis was once a streetcar town. Electric streetcar routes criss-crossed the city and the inner-ring suburbs, serving a wide swath of St. Louis County, as well as selected portions of the Metro East, particularly via the interurban car network across the McKinley Bridge.
As the name suggests, most streetcars ran on streets; but some ran on private rights-of-way. And in some cases, particularly in the suburbs, access to those lines was available not just via traditional streets, but also by semi-public walkways running perpendicular to neighboring streets.
Some of those rights-of-way and walkways still exist. If you walk around and look closely, you can find them.
Most of the walkways I know are in or near University City and Clayton. There used to be a walkway that provided access to the 93 Lindell line and the Rock Island Railway (now Forest Park Parkway - or, actually, the under construction MetroLink line) called "Waterman Way" in the Skinker-DeBaliviere neighborhood from about 5900 Waterman to 5900 Forest Park Parkway in St. Louis City, but it was vacated and barricaded in 1999.
Identifiable former streetcar rights-of-way:
Since at least the 1970s, DeMun has been closed to through traffic at a point between Arundel Place and Northwood Avenue, next to Captain Elementary School. Indeed, part of the old street is now the school parking lot; and walking through there requires going up several steps. This enforces the separation between the single-family large-lot residential district north of the school, and the DeMun Ave. retail strip accessible via Clayton Road to the south. Thus, DeMun no longer has public transit service.
Likewise, Yale Ave. was broken by the construction of Highway 40 through Richmond Heights in the 1960s. The wide median is still evident on Yale on an orphaned half-block from Wise Ave. to West Park Ave., and then south of 40 from Nashville Ave. all the way to Manchester Ave. in Maplewood. I suppose that the connection between DeMun and Yale on this route was once made on a right-of-way that's now part of the St. Mary's Health Center campus. Bus service still operates just one block west, on Bellevue Ave., via the #16 City Limits.
Still open walkways to old streetcar stops:
Other secret passages that are fun, but probably not streetcar-related: