You might have heard recently about city officials closing the Tucker Blvd Bridge in downtown St. Louis. Calling the structure in question a bridge is a bit inaccurate. They’re talking about an old commuter railroad tunnel/viaduct that runs under the street as far as Washington Ave. Here is a great site with pictures.
I found the exit to the tunnel, north of O’Fallon St. I wish they could figure out some way to make this tunnel useful for MetroLink some day.