The two engines, A4 4489 Dwight D Eisenhower and A4 60008 Dominion of Canada, were unveiled last weekend at the centre.
Visitors saw the locos in a genuine working engine shed environment before their return to America and Canada next month.
Alexa Stott, marketing manager at the roundhouse, said that hosting the two locomotives was an honour.
She added: “The visit of the two A4s has cemented our reputation as a premier venue.
“These are two unique, stunning locomotives and they have only been able to come to this roundhouse. It took 12 months of negotiation for us to make it happen.
“It’s massive for the area – we have had thousands of people come from all over.”
The roundhouse, which is the last surviving in the UK with an operational turntable, attracts more than 30,000 visitors a year.
During the event, the two A4 locomitives lined up alongside the A2 Blue Peter, as well as the A1 Tornado and the A4 Bittern.
The A4 Bittern was also presented with a plaque last week commemorating its historic 90mph runs in 2013.