I'll let [@Eviledd1984] say for sure... but I took his post as Sergio headed toward the [i]front[/i] of the car with Misti & Taylor.  (Also where Bobby and Sparrow Hawk are trying to gain entrance to the express car, which is directly in front of the passenger car.)

The rider on the brown and white pinto did indeed back off, but was not the one to yell.  The one who yelled was a dark haired man of average height (the same that Sergio recognizes!)