You're asking a question nearly impossible to answer with any accuracy.
What do you mean by up to speed? Up to speed might be crawling at Restricted Speed or 60 MPH depending on territory speed limit, signal indication, best speed for tonnage/grade, or any number of other factors. How heavy is the train and how much power do you have? Two GP9s or five AC44s? Good traction control systems on a DC engine or advanced AC spin control? Are the engines in good condition or are they tested low/de-rated, blowing fire out the stack or have fuel rack problems? Even 1.3 miles is still not enough info. Are they 100 ton cars or 130 ton cars? There are plenty of other factors too.
Wheel slip speed? You can get wheel slip in any notch. What are the rail, weather and locomotive conditions? Cold/hot, wet/dry, leafy, bug coverage, presence of lubricators, well maintained engines or junk, stick rail or CWR? Hard curves or tangent? You might be in run 8, dumping sand and moving at less than one mile an hour, with or without wheel slip. And wheel slip in practical terms means something different than design terms. Most all modern engines have a certain amount of wheel slip built in before they will indicate a slip. To the designer the engine is slipping any time wheel speed exceeds ground speed, to the locomotive engineer the engine isn't in wheel slip until it goes to free-wheeling, indicates, and he has to take action.
Without knowing the territory or what the train you saw was up against, I can only guess that they started on a less than favorable signal or speed restriction and by the time you saw the helpers they had better and had throttled up, but that's just a SWAG. As for 'going to run 8', depending on the variables above, you might never go to run 8, or you might push the brake handles to release and pull it right out to 8 and let 'er go.