@animus - the cave in might not be isolated. If it happened in several areas or in a larger area than what we could see, it makes sense that it might take a week before they are found. They might have just been really unlucky.

@Alhuin - People have been known to survive worse for longer. As for it suddenly going dark, if you recheck the episode, the lights just went off outside the train when the trained started shaking, which obviously means they are artificial. That means they are underground at that point and the lights were the tunnel's illumination that got cut due to the incident.