If it's clearly a suicide or tresspass, then the police have very little to do, other than helping with the clear-up and of cource that lovely job of informing the family. If foul play is suspected though things are very different, and natually the area becomes a crime scene and efforts have to be taken to preserve evidance. Whilst in that sort of case, the inconvienience and cost is a pain, it's important to think what it would be like if it was your loved one who had been killed say by being pushed infront of a train, and the murderer couldn't be brought to justice because vital evidance was lost in the rush to get the line re-opened. Thankfully that's rare though, and most fatalities are clear cut suicides or misadventure. Of cource though, with any death, it has to be treated as suspect until proven otherwise, and rightfully so. I doubt passengers would be held on board by the police for an off train fatality though (if the fatality involved a train passenger it's a different matter), but oviously if the train itself is detained between stations, getting passengers off isn't easy. It's even worse than a breakdown, since consideration has to be taken both of preserving evidance, and making sure passengers being detrained don't see anything nasty (e.g you can't just walk them back to the previous station where the body is still scattered around the tracks).
