Recent discussion here described what's going on. Search for "Comet cars" for more details. To summarize:

1) A group of old New Jersey commuter cars is being refurbished for Caltrans by Amtrak. When they arrive, they will replace 1 or 2 of the current San Joaquin sets (hopefully with enough cars that the train capacity is maintained - the cars hold about 2/3 the passenger load of a normal Cal Car). Think of them as old-new versions of the Horizon cars once used on the SJs.

2) The Cal Cars freed up by the NJ ("Comet") cars will be spread around to increase the length of existing (SJ?) and Surfliner trains.

3) The Comets on the SJ are *supposed* to be temporary until the Real New Cars start arriving in a few years, at which time everything will be reshuffled again and the NJ cars will become extras.

4) The new cars (coming in a few years) are based generally on the Cal Car design, but with tweaks for nationwide use. Most likely, they will have smaller luggage areas similar to other Amtrak short-distance equipment. This is speculation - I haven't seen detailed information about the new design and don't have time for extending googling at the moment.

I suspect the immediate luggage thing is related to the Comet cars. They don't have luggage lockers overhead like the Cal Cars - they have open racks like you used to see on buses. The racks aren't very big; hence the likely limits on carry-on size.

I wasn't aware that 2 new locomotives were being bought too. Replacing the Caltrans P32s?