Re: Caltrain electrification on track: Draft environmental impact report released for modernization plans
Some properties use EMU's, some locomotive-hauled (electric or diesel). It appears Caltrain is going the EMU route. The deciding factor seems to be quicker acceleration / braking to allow more capacity in the system. The following video from the website gives a good overview.
Caltrain Video
Amtrak is acquiring new electric locomotives for service on the NEC to handle their existing fleet (and some new) coaches. A number of their NEC trains travel "Off Corridor" and switch between diesel and electric. The original Metroliners from the late 1960's were EMUs but it appears locomotive-hauled trains make more sense for them.