David Lindsay's "North british Railway Register of Stations, Routes & Lines" gives the 1 July 1848 date without comment.
If it's the right one, there's a report in the "Caledonian Mercury", 1 October 1849 page 3 column 2, about Queen Victoria's rail journey south from Balmoral that includes:
"The company's works at St Margaret's were splendidly decorated, three triumphal arches having been erected over the line within a short distance. About 6O or 70 flags waved from the roofs and chimneys of the workshops, and on the gable of the fitting workshop was a floral crown, with the letters V. A. and the word "Welcome." A little farther on, the royal standard waved proudly aloft, and beside it were two crowns, an ornamental locomotive wheel, and an anvil, all composed of flowers. The boys of George Heriots Hospital were accommodated on a platform, where also was congregated a large assemblage of ladies and gentlemen. The wives, children, and friends of the workmen filled a long train of trucks parallel with the line, and the workmen themselves acted as special constables, and maintained excellent order.
"At the Portobello station, a grand triumphal arch was erected, which was gorgeously ornamented with flowers from Dalkeith Palace Gardens, and with evergreens and flags. Here the royal party were again saluted by multitudes of the inhabitants and visitors of this flourishing watering place. The utmost liberality was displayed in the admission of the public to the station, and there might be present not fewer than 5000 or 6000 people, including many from Edinburgh. An extensive line of about 50 trucks were ranged near the station, and afforded accommodation to the vast assemblage...."
It's noticeable that the boys and others are simply "accommodated on a platform" with no further detail whilst Portobello is clearly named and referred to as a station to which the public are admitted for the occasion. It is not clear that "a platform" indicates even a closed station, let alone an open one.