Two major problems: First, the Coast Starlight is habitually late so the midnight departure from Sacramento may be more like 2 or 3 am. And, depending on what “around Thanksgiving” means, getting seats, much less seats together, may be a real problem.
Amtrak has a history of overbooking trains. If your friend's trip date is on one of the peak travel days (Wednesday 11/24 and Sunday 11/28), the odds of that happening increase dramatically. The train will fill in the Bay Area, so it is possible that your friends will board only to find they do not have seats. Unlike the airlines, Amtrak suffers no federally-mandated repercussions for this behavior. The option then offered is to spend the night in the lounge. Not charming. Even if there are seats, getting seats together may be an extreme challenge. Seats are not preassigned and, by Sacramento, the passengers will be sleeping are getting someone to move will be tough. I personally would not risk it. So, I have three suggestions:
One: Follow Alan’s advice and book a sleeper. Your friends will get assigned rooms and a nearly 100% assurance of having accommodations when boarding the train even late and even in Sacramento. Downside: booking sleepers takes an inexpensive trip and moves in squarely into the very expensive category. If it works financially, and if rooms are available: great. If not, lets go to the second option:
Second: make the seeming illogical move of taking an earlier train from SAC to Oakland and board the northbound Starlight there. The northbound Bay Area passengers from LA will be leaving the train there, but the passengers boarding for points north will get on primarily at Emeryville (the next stop), so getting on the northbound at Oakland should make the seating choices wide open. And, even if the train is running late, boarding will be at a far more civilized hour. Of course, this will also bump the trip cost up, but not by nearly as much as booking rooms on the train. Still no good? Well, there is also option three:
Third suggestion (railfans take cover): Fly. Avoid the entire Amtrak Thanksgiving hassle and take a plane. Seats are assigned and denied boarding is compensated. Fares are about $150 one-way (Alaska). And 90 minutes after leaving Sacramento your friends are in Portland (the Starlight is lucky to get in less than 90 minutes late). Don’t get me wrong: the train ride is a great experience. I am just not sure that this particular itinerary during Thanksgiving provides much of an opportunity to enjoy the trip.