Hutch: staying with diesel would work as long as trains still terminate at 4th St. in SF. But someday (if the HSR money holds out) the tunnel to the Transbay will be done and then no more diesels allowed.

Max: I wouldn't rule out dual-powers for schedule reasons. The older ones in the NY area (P32DMs, FL9's, whatever LIRR calls theirs) may not have any more power on 3rd rail than they do with the diesel running, but the recent purchases at NJTransit do have more power on electric than they do on diesel (5360 hp electric v. 3600 diesel at the rail - [en.wikipedia.org] ), and both are high enough to move a normal commuter train (4-6 cars) along at a decent pace. Also, the Gilroy trains in the current schedule run limited-stop north of San Jose, so there are fewer occasions where hard acceleration is needed. Finally, with the number of stops on the Peninsula, any kind of commuter train really doesn't need to go more than about 70 mph - stops are close enough together that any kind of rail equipment would have a hard time going faster before hitting the brakes for the next stop - so with careful scheduling a train with a high-enough-powered loco could stay out of the way.