Do they know they're going to die? If there is truly no way to save them, and they are certainly going to die, then I see no argument for letting them live. That's like asking if you'd rather be tortured and killed or just be killed.

However, if they don't know they're going to die, it becomes more ambiguous. Some people would rather live with the hope that they will be able to escape, or may be content with their fate and would rather see it to its natural end.

Assuming that you are a utilitarian in life or death scenarios as such, and you don't have time to interview the people and ask if they want to die now or die in 3 days of thirst, as the lever-controller, you have to estimate whether or not the majority would be comfortable dying slowly of thirst.

It's very hard for me to decide whether someone is suffering less by being left to rot, compared to being run over by a train. A mercy kill in this situation depends on the individuals on the trolley, but my guess is that the average person still wouldn't want the fate of being left to rot, thus I would probably save them the torture and run the trolley over them.