i was doing the Mixed freight to Whitefish activity, and saw hikers alongside the tracks in places. I thought MSTS didn't have people.
It's something that can be added to a route as a static object. With a clever hack, you can even make people move (basically, make a 3D object of a person, tell the sim it's a car, and use a carspawner object to animate them over a "road" textured to look like a path or something), but for the most part, it's just a simple model stuck in there (like a station).
I know the Activity Editor includes a slider for "persons" as a hazard, but unlike the "animals" slider which adds deer along the tracks, the "persons" one doesn't visually do anything (and it's unclear if it does anything at all).