|First day celebration in 1961|
Until the late 1970s, most mail in the United States was carried on trains. While in transit, mail was canceled on these trains using rubber stamps denoting the railroad.
In 1961, the Washington Park and Zoo Railway became one of the first recreation railroads to have its own cancellation stamp. With the demise of the Railway Post Office, the zoo railway is the last operating United States railroad with its own authorized railway postal cancellation to continuously offer mail service.
The locomotive of the Zooliner has a postal mail slot on the side of the cab, and mail boxes are located at the Zoo and Washington Park stations.
|First day cancellation |