History
Texarkana's Federal Building straddles the Texas–Arkansas state line
The United States Post Office and Courthouse in downtown Texarkana was built in 1933 and is listed on the National Register of Historic Places — it is the only federal building in the country that sits in two states at once.
State Line Avenue in Texarkana runs right along the border between Texas and Arkansas. The Federal Building sits directly on that line. It holds a post office and federal courtrooms serving both Texas and Arkansas. No other federal building in the United States sits in two states at once.
The building was finished in 1933 and is listed on the National Register of Historic Places. The first-floor post office, third-floor courtrooms, and judicial offices are still in use today.
This building shows Texarkana's split identity in a concrete way. The city was founded on December 8, 1873, when the Texas and Pacific Railway and the Cairo and Fulton Railroad met at the state line. The two sides of Texarkana have always been separate cities — each with its own mayor and city council — while sharing some services.
Source to confirm: GSA: U.S. Post Office and Courthouse, Texarkana