if you want them to have the murder mystery experience i would do one of the venues in san francisco or old town sacramento. if you want them to have the experience of the wine train i honestly believe it's better to have them do the lunch or brunch. my reasoning is that when you do the train ride at night it's pitch dark outside and you don't get the vineyard views which for out of town visitors seems to be very appealing.
having grown up in napa i can honestly say that esp. if your visitors are traveling in the next couple of months they would realy enjoy a daylight ride on the train so they can see the valley (i love the fall colors but equally love the wintery misty fog laden vineyards).
