Two black men, their hands gripping suitcases, make their way down a tree-lined road. A camera catches only their backs — the hunched shoulders and purposeful stride.

They are northward bound.

The story of their journey and that of hundreds of thousands of other Southern blacks early in the 20th century is a watershed moment in U.S. history.

From about 1915 to 1970, 6 million or so African Americans moved from the South to the North, leaving behind mostly rural farming existences for industrialized cities where they hoped to find opportunity and, generally, better lives.

Many made that journey via passenger and freight train, so it’s no surprise that Amtrak has chosen it as the subject of an exhibit. The show will go up in Washington’s Union Station next to Gate A on Saturday and will remain there until Sept. 26.

The images and documents chronicle the beginning of the migration wave, up to 1930.

“We always try to have cultural exhibits to show the diversity of the impact of trains and people who have impacted the railroad system in our country,” said Darlene Abubakar, director of national advertising for Amtrak. Amtrak has also produced exhibits on Mexican and Asian contributions to the railroads.

The Great Migration exhibit was planned for National Train Day in May and was first displayed in Philadelphia. The response was so positive, Amtrak said, that it decided to take the exhibit on tour.

Allen Ballard, professor of history and African studies at the University of Albany, worked as a consultant. In two books, “One More Day’s Journey: The Story of a Family and a People” and “Breaching Jericho’s Walls,” he tells how his paternal grandparents and their eight children left South Carolina and moved to Philadelphia in 1917.

“I was basically trying to figure out who I was,” said Ballard, 80. “It was a bittersweet emergence into the North with opportunities galore and lots of setbacks. Sometimes families came to the North looking for the ‘Promised Land,’ but ‘Promised Land’ turned dark for them.”

The Great Migration is an example of what researches call the “push-pull theory”: things always push people out of a place and others pull them in.

“Jobs vanished in the South,” Ballard said. “When America went into war in 1917, there was a cutoff of immigration from Europe to the U.S. In the North, you had therefore not enough laborers, and this created kind of a suction effect, a need of hands to work in the factories. They were pushed out by economics and by oppression, and they were pulled into the North by opportunities, there were jobs.”

Amtrak called upon the resources of a number of institutions for the small exhibit, among them the Schomburg Center for Research in Black Culture, the African American Museum of Philadelphia and the Library of Congress. The Henry Ford, University of Chicago and Florida State archives also played a role.

“It’s small in scope, but it’s enough so folks can begin to understand what it was like, all of a sudden, to be a stranger in a foreign land. That is a universal experience,” Ballard said.

The show’s final stop is Baltimore’s Penn Station, where it will display Sept. 27 to Oct. 26.