> And tbh, I just can’t see which sort of emergency can happen inside the train that would require an immediate full stop.

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.