The Detroit-Superior Bridge in Cleveland (so named because it connects Detroit Avenue on the West Side with Superior Avenue downtown) was opened in 1918. While renamed a few years ago to the Veterans Memorial Bridge, to most it is still the Detroit-Superior Bridge.
When it was opened in 1918 it had streetcars running on the lower level with the cars, buses and trucks on the upper level. (photo below is from about 100 different internet sites). When the streetcars stopped running in the 1950s, the lower level was closed off.
Every once in a while the Cuyahoga County Engineers Office will open the lower level for tours. With the last tour 4 years ago the open house this year was very popular, with an estimated 10,000 people checking it out.
The outer walkways were only partially open.
The steel frame allows views down to the river, almost 200 feet below.
On the west side, the abandoned West 25th Street subway station was open.
There have been numerous proposals for use, including bike/pedestrian trails, etc.