In April 1975, the American Freedom Train set out on a tour across the United States to celebrate 200 years of American independence.

On-board were more than 500 priceless artefacts, documenting important moments in America's history - including an original copy of the Constitution, Thomas Edison's first working light bulb and a NASA lunar rover.

Over the next 21 months, seven million people visited the travelling museum as it made its epic journey around 48 states.

Jacqueline Paine speaks to former train security guard Lou Nelson, about taking America's history to the people, as the country prepares to celebrate its 250th anniversary.

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(Photo: People in Archbold, Ohio queue to see the American Freedom Train, June 1975. Credit: AP)