Portillo, who presents the BBC TV documentary Great British Railway Journeys, will wave his green flag and blow a whistle to open the permanent show.
The exhibition features graphics, videos and hands-on interactive displays along with railway memorabilia and models.
The opening of "The Dream that became Steam" exhibition, takes place on the afternoon of 23 May, when the garden's azaleas and rhododendrons and many flowering trees and shrubs in the 60ha woodland garden will be flowering.
Leopold de Rothschild, whose family owns the gardens, is behind the Exbury Garden Railway, which opened in August 2001.
"The railway has been more popular than we ever dreamed," he said. "We believe this permanent exhibition will be of interest to many of our visitors and we are delighted that Michael Portillo has agreed to come to Exbury to open it officially."
The exhibition will be open to all those who buy a ticket to ride the railway, having entered the gardens.
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