first, about cars, NJT still has more Comets, so if one thinks the stainless steel shels are worth refitting as was done w/the two trainsets currently in service on the San Joaquins, then another two trainsets should be possible long before the Siemens deliveries.

Second, as to the markets, and what sort of amenities. There are folks who actually prefer a full night's sleep enroute with an arrival early enough to get to the dreaded meeting or a pleasant day in the destination city. Could such a train fill a sleeper each to SF and Sacto via Oakland? Maybe. Would need to be "slumbercoach pricing" no full diner meal perks, beds all made down, ready for use, but have a shower stall so one can arrive "presentable".
Which brings one to commenting that cutting off one sleeper and one coach to be hauled to SF on the next Caltrain should be possible. The rest of the train in effect becomes a Capitol Corridor run as it leaves SJ. (and of course, the reverse SB) In order to make the process work well, some speedup along the Coast Route is necessary.

Would it fill up? Don't have any proof either way. OTOH, the several CA corridor services are some of the highest ridership in the US...

Might as well buy the rest of the Comets anyway, only, keep the electric sliding doors and contemplate highlevel platforms.