Let me try to illustrate the number of train sets... this is based on observation and by no means scientific... feel free to correct as needed.
San Joaquin:
702: From Train 703 Overnight Layover SAC -> BFD Turn to Train 715
712: From Oakland Yard OKJ -> BFD Turn to Train 717
714: From Oakland Yard OKJ -> BFD Turn to Train 703
716: From Oakland Yard OKJ -> BFD to Overnight Layover
704: From Train 701 Midday Layover SAC -> BFD to Overnight Layover
718: From Oakland Yard OKJ -> BFD to Overnight Layover
711: From Train 718 Overnight Layover BFD -> OKJ to Oakland Yard to Maintenance or Train 716
701: From Train 716 Overnight Layover BFD -> SAC to Midday Layover to Train 704
713: From Train 704 Overnight Layover BFD -> OKJ to Oakland Yard to Maintenance or Train 718
715: From Train 702 Turn BFD -> OKJ to Oakland Yard
717: From Train 712 Turn BFD -> OKJ to Oakland Yard
703: From Train 714 Turn BFD -> SAC to Overnight Layover
With that said... all trains sets would operate to Sacramento at some point with the exception of 712/717. A minimum of 6 trainsets would be required to operate the route, however that would not allow for maintenance time, much less time to add/remove cars or change the consist. The 7 trainsets would make sense for the route. There are 6 Coach/Baggage cars, however with the Superliner Rebuilds the flexabilty of car assignment has increased as each San Joaquin has a Coach/Baggage car, while Capitol Corridor do not require one, although they will from time to time when trainsets are changed around.
A typical San Joaquin Consist is: Coach/Baggage, Coach, (Upstairs) Cafe Car, Coach, Coach/Cab Car - 4 cars with coach seating, 3-4 cars with lower level seating depending. Unfortunetly as of recent, a coach has sometimes been removed from the consist leaving Locomotive, Coach/Baggage, Coach, (Upstairs) Cafe Car, Coach/Cab Car. These trains either sell out or are uncomfortably crowded, people seated in the Cafe Car, and overstuffed with baggage during the midday travel period and on Fridays and Sundays.
Nightly Northern California Trainset Locations:
Auburn: 1 CC Trainset Layover
San Jose: 1 CC Trainset Layover
Bakersfield: 3 SJ Trainsets Layover
Oakland Yard: 2 active CC Trainsets Overnight in Yard, 2 active SJ Trainsets Overnight in Yard , spare train sets for CC/SJ (+1 CZ Trainset Turn/Layover and numerous private cars...).
Sacramento: 4 CC Trainsets Layover, 1 San Joaquin Trainset
Total Minimum Active Trainsets Daily: 8 CC, 6 SJ Minimum with another in Yard - Total Minimum Need: 14 Trainsets on Weekdays, 13 on Weekends
A typical Capitol Corridor Consist is: Locomotive, Coach, Coach, (Upstairs) Cafe Car, Coach, Coach/Cab Car
Alternatively another Capitol Corridor Consist used is: Locomotive, Coach, Coach, Coach/(Downstairs) Cafe, Coach/Cab Car
They always try to have 4 cars available with coach seating whether it is a 4 car or 5 car train.
Both routes use 6 car trains on Thanksgiving Week with the following exception:
Day Before Thanksgiving incoming trains 521 and 527 are coupled together to form a 9 car train for train 532 to SAC and train 545 to OKJ. Generally half is sealed off for train 532 as the extra cars are mainly for train 545 with a flood of people and UC Davis students returning back to the Bay Area around the same time. To make up for the cars lost to the longer train (from train 527) trains 542 turning to train 551 use a 5 car Caltrain Train set with limited snack service. All available cars are pressed into service to make the longer trains so the Oakland yard and shops are virtually empty leading up to and the days following Thanksgiving.
It is a lot of information I think I wrote out more so so I can understand what's going on in my head. Cars are stretched thin so any additional equipment would have a use. I know it is similar throughout the rest of the Amtrak system, but just looking at Amtrak in Northern California, the cars are there in a perfect world. But with BO Cars and PM's in addition to unschedule maintenance, trains often run with fewer cars than normally scheduled.
And as a sidenote: I was asked by someone about the Surfliner cars in Northern California. The cars are mix and matched in the existing consists. There was one consist that used to have a Surfliner Coach, Surfliner Coach/Cafe, and Surfliner Coach/Cab car in addition to one California Car Coach, but that set has since had the Surfliner Coach/Cafe replaced by an Amtrak Superliner Coach/Cafe breaking up the set. It is all mix and match with sets generally having 1-2 Surfliner cars at most if they have any at all.