If, in the old days, the conductor didn't punch your ticket, you could reuse it, right? Same thing here. Surfliner, like Capitol Corridor, may have segments where there's just not enough time between two stops for everybody on the train to have their tickets scanned, if there's a large crowd. So occasionally somebody doing a short, one-stop trip, might get a bonus. The tickets do expire (in a month or 2?), though, so I wouldn't wait too long to reuse it.

Of course, if it was a round trip and you got scanned the other way, shazaam, computer takes care of cancelling the missing half of the trip.

As for other itineraries, it's rare (though possible) for users of Surfliner or the other California trains to connect to a long-distance train using their Amtrak ticket. Thruway bus perhaps, but the bus is a separate ticket that the driver scans or collects when you get on. Presumably, Amtrak's computer system could catch that and cancel your connecting train ticket, too. If not now, soon, considering that Amtrak now has a Delta airline CEO. The only other "itinerary" would be something like using Metro in LA with your train ticket (works with Metrolink, not sure about Amtrak; on the Cap Corridor trains you have to buy a discounted BART ticket on the train or get a transfer from the conductor to use local transit as part of a train trip).