I've taken the Pacific Surfliner southbound from Santa Barbara to Anaheim or LA maybe 3 times. Once was actually San Luis Obispo to LA after the 2022 Southwest Airlines Christmas meltdown, so it wasn't exactly from Santa Barbara. I think I've taken the Pacific Surfliner northbound to Santa Barbara maybe 4 times. I think once from Anaheim. A lot of these were connected trips from/to Northern Califonria with buses, but you probably don't have to worry about those other than the possibility that a train might be held up waiting from a late arriving bus.
It's actually quite easy. For coach travel the Pacific Surfliner is considered an "unreserved" train, which is kind of a misnomer. If you buy a ticket you have a reservation and you'll be on the conductor's manifest for that particular train. But you theoretically have the flexibility to take any train going between the same endpoints (or even within the endpoints) within one year of the date of the reservation. There are some exceptions if there are discounts. I've boarded an earlier train on the same day or a later train on the same day. Examples would be that I had a ticket from Anaheim to Santa Barbara for the evening, but I boarded a different train about 4 hours earlier and spent more time in Santa Barbara before catching the bus home. I've also gotten the same price (in Amtrak loyalty points) for Anaheim to Santa Barbara than LA to Santa Barbara, but we boarded in LA. I figure that would give me some flexibility if we just stayed in the Anaheim area.
There is no reserved seating on the Pacific Surfliner and there is a nonzero possibility that seating will be difficult to come by. Business class is reserved, and there's no chance of being overbooked. That being said, I wouldn't worry about it. There's room in the cafe car, where they'll ask people to sit if there's a lack of seating in the coach sections.
The other thing is getting discounts. Children are theoretically free under the age of 2. I've talked to some conductors who mentioned the "oldest looking 1 year olds I've ever seen". There's also a 50% discount for children 2-12, but if they're teens they won't be eligible. But they should be eligible for other discounts. They eliminated the 10% advanced purchase
AAA member discount. But if you're going to Anaheim, there should be a 20%
Disneyland discount, although it's theoretically just a discount if Anaheim is the origin or destination and the promo code is entered. You'll also be able to use your train ticket for a complimetary ride on ART, although I'm thinking if you're already going to be spending a few days in the area you're going to already be getting an ART pass. Also - ignore the info on there being any ART "line". They no longer exist for any route to the Anaheim train station. Any trip to/from the station will be what they call an "on-demand" ride using the A-Way-WeGo app.
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The app takes a little bit of practice to use. It was really weird at first. I suppose it's similar to requesting an Uber ride, although they'll try to link as many passengers together as possible. It's not quite as flexible though, as there are designated points where drivers can pick up or drop off, but you don't really know where these stops are. I've ended up playing around with locations to get it to work, and there are a lot of tricks needed to get it to work. But once you figure it out, it becomes easy. But initially it's bizarre. Also - as far as I can tell they've discountinued ART service in Buena Park, which was really cool. There might have been three stops (Knott's Berry Farm, Wax Museum/Medieval Times, and Fairfield Inn) in Buena Park.
I can answer any questions on ART or Amtrak. However, I may just know enough to be dangerous.