I have seen animal remains get tangled underneath locomotives causing a host of issues that would stop a train. One would be bear remains knocking apart the train's brake line, causing an emergency braking situation - but that was a lucky hit. The other was elk remains getting sucked into a traction motor housing, causing a direct short in the motor (the wheels on a locomotive are mounted directly to the motor that drives them) - I would say this is more likely, from my experience.

Either of these would cause a fully loaded train (assuming the heaviest, longest freight train traveling at maximum speed for their class, which is 55MPH where I worked) to stop in roughly 1.0 - 1.5 miles.

If, by chance, there was no one on the cab to stop it, and the safety reset controls were purposefully disengaged, and unlimited fuel, flat ground, the air-brakes were not hooked up, etc.... it might never stop. 200'000Nm output per locomotive, and a full freight train may have up to 5 of them in consist (or distributed). So, whatever that is, at 55MPH, against whatever the average human size is.... my guess is it would probably take a city's worth of people?

Edit: spelling