I wouldn't trust AI to be an expert on anything South Park.
I suspect fouling points weren't marked by poles or painted rails on the pre-1885 South Park, if at all.
There exists an 1885 UP standards book showing no fouling point posts nor signs but it does include a "clearance plate" for affixing to the top of a tie.

"To be used in principal yards."
The existence of the above clearance plate in a UP Standards book should not be taken to mean they were used on the Colorado narrow gauges under UP control. You
might see these in Denver, possibly Golden or Boulder. The South Park by 1885 wasn't a money-maker for the UP. Maintenance was kept to a minimum. Expenditures on new equipment was pretty much limited to flangers and the rotary plow for the rest of the decade and into the mid-1890s. A lot of equipment had been moved out of Colorado to the U&N and OR&L by 1885, two lines which did make money.
People need to look closely at photos to see if they can find any examples of these plates. I'll say my guess is not, just to improve the odds of finding one.
Dave Eggleston
Seattle, WA