Camilla's necklace boasted 36 rubies interlaced with diamonds

They boarded a private electric train for their journey to the city, along with academics and policy makers.

The group spent the trip discussing how to revitalise urban areas.

Prince Charles is due to pick up an environmental award from Harvard Medical School's Center for Health and the Global Environment.

He is scheduled to give a speech on environmental issues and will receive the Global Environmental Citizen Prize from former Vice President Al Gore, who won it last year.

Prince Charles and Camilla visited New York's Harlem Children's Zone and saw several social service, health and educational projects.

A meeting with schoolchildren and their families to watch a maths carnival, basketball practice and rehearsal for Shakespeare's A Midsummer Night's Dream were also on the itinerary.

Charles and Camilla's decision to travel to New York by train, rather than take an internal flight, is seen as an attempt to appear more environmentally-friendly.

The couple arrived at Penn Station after leaving Philadelphia's 30th Street Station in three vintage carriages, powered by The Pennsylvania Railroad Business Car 120 locomotive.

It is the same one that once transported John F Kennedy to a football game, and later transported the body of his brother Robert Kennedy to Washington after he was assassinated.

Civic reception

On Saturday the royal couple were welcomed at a civic reception in Philadelphia by the governor of Pennsylvania, Ed Rendell where 400 people gathered outside.

Charles and Camilla also saw the city's famous Liberty Bell, with its distinctive crack down the side.

They then attended a white-tie concert at Philadelphia's Academy of Music where the bill included rock star Rod Stewart.

The duchess wore a spectacular necklace - 36 rubies interlaced with diamonds - which Clarence House said had been presented to her while she was accompanying Charles on other recent overseas duties.

But they refused to confirm who gave her the generous gift.

Camilla teamed the rubies with a deep red velvet Robinson Valentine floor length evening gown.