I think some of these cars will be for wreck replacements. Some might be used to expand the size of existing trains, especially on weekends and holidays.

Another issue Amtrak has is they rarely have a spare trainset at a terminal. The extra set they bought is usually the set that is rotated through the shops for maintenance. The sets that laid over in San Diego used to rotate out at Los Angeles with one set going up in the morning to LA, then to the shops, and hopefully back that evening. The Santa Barbara/SLO sets sometimes came into San Diego once a week from LA as a double set. Going out they would load passengers in the opposite set and drop the old set at LA for maintenance. Remember that the equipment rotations aren't only for cleaning and inspections, but the LSA's stay with the same car for its entire rotation since the inventory is signed out to them and their personal responsibility (as in they pay for any shortfalls). FDA requirements mean the food service cars have to be fumigated on a regular basis, and cab cars are always taking a beating at crossings.

It gets real interesting when a couple trains get jammed up or damaged and LA has to come up with a trainset in a hurry to get a train out on schedule. The Superliners do not have the same capacity and have manual doors, but have been used.