I don't know about London, but I'm one of the small percentage of Americans who have commuted regularly on a train. People have an obnoxious tendency to decide to commit suicide via being intentionally hit by a train. This stops the train and probably blocks the tracks for trains that need to pass through for some time until the police have done their thing; unlike with cars, trains can't just drive around the incident on some nearby road.
Not to mention that I imagine that it's traumatic for the train driver.