Trains are multiple vehicles that are somewhat loosely connected. A train segment could be detached from the rest of the train.
https://abcnews.go.com/US/amtrak-cars-separate-boston-bound-...
And other mechanical issues with the train besides (for instance: the train being on fire).
The fact that you can't think of something doesn't mean that you have done an exhaustive search.
Trains are fail-safe. They only run because the brakes are held open by a pressured air system running along the whole train. In case of a detachment, the hose connecting the wagons would break, releasing the air pressure and thus slamming the brakes down.
The Eschede crash in Germany: https://en.m.wikipedia.org/wiki/Eschede_train_disaster -- part of the wheel came through the floor into the passenger compartment. The passenger couldn't find an emergency stop, and by the time they'd found a conductor and got their attention (which took minutes) the train crashed.
Trap-and-drag accidents where somebody on the platform gets clothes trapped in the train door and is pulled along with the train as it starts to pull off. If you don't emergency-stop the train within seconds that person is likely to suffer serious injuries or die. Likely the conductor/guard isn't near that door and only passengers will have seen the problem.
And tbh, I just can’t see which sort of emergency can happen inside the train that would require an immediate full stop. Even if something life threatening is happening inside, the best option is to call and meet the emergency services in the next station.