FREMONT — After the Pony Express and before Federal Express, when mail absolutely, positively had to get somewhere, it went by train.

From the 1860s to the late 1960s, the U.S. Postal Service operated a fleet of rail cars that effectively were post offices in motion with clerks sorting mail coast to coast.

On Saturday, the Postal Service and the Golden Gate Railroad Museum are teaming up in Fremont to bring back a piece of post office and railroad lore.

A refurbished railway post office car will roll into the Niles district with a full staff of postal employees, and several retirees who worked as mail clerks on the City of San Francisco streamliner, which transported people and mail from Oakland to Chicago.

The car will be an official post office for the day.

Residents are invited to bring their mail, which post office workers will mark with the streamliner's historic cancel stamp.

The event marks the 60th anniversary of mail service on the City of San Francisco, which stopped transporting mail in 1967.

The railway postal car to be displayed is one of only about two dozen left in the country, said David Roth of the Golden Gate Railroad Museum, which owns the car and stations outside Sunol.

"It was a huge human sorting machine," he said. The cars consisted of up to 15 postal workers, some of them armed as the cars delivered coins to casinos in Reno, Roth added.

The car will be on display from 10

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