After reading the complaints that there aren't enough historical posts to go around, I finally found a legitimate primary source to share.
I found on Utahrails.net the exact day that the first Rio Grande train entered Ogden over the third rail on the Central Pacific tracks, and then proceeded to look up the Ogden Standard's issue for that date.
"THE D. & R. G. R. R."
May 17, 1883
"Notwithstanding all the opposition made by rival roads the 'Baby Giant' has accomplished its object. The first passenger train of the D&RGRR, consisting of engine, two coaches, a pullman sleeper, and baggage car, left Ogden, this morning, at 9:47, for the South. Owing to the fact that the bridge over the Weber River was not completed, the passengers and baggage were transferred by carriages to the other side of the river;but this evening's D&RGRR train from the South was expected to come right into Ogden, at about 5:30, when the ovation which was talked about, some time ago, was to be given the plucky, indomitable, and irrepressible 'Baby' road."
I am assuming that "rival roads" refers to the UP, as the Central Pacific was much more welcoming to the D&RG.