For reducing emissions then maybe battery locomotives would make more sense? Wabtec has a working one:
https://www.youtube.com/watch?v=6_mpAQ3s_DU
I wonder if it would be possible to construct a battery car that can connect to an electric locomotive through a conduit. They could be hitched to conventional diesel-electric locomotives that have the ability to switch their motor's power source to the battery car.
A siding with room for many battery cars would be built in Barstow. Trains going to the Port of Los Angeles would pick one up so as to run electric only in the LA Basin. Trains leaving LA and heading towards the Midwest would drop off the battery car to be charged and turn on their diesel generators for the long trip across the plains. It's okay if the battery cars take a while to charge while parked on the siding, because there would be several of them to guarantee a "fresh" unit is available. It would be simpler and cheaper than electrifying a lot of miles of trackage in LA proper that wouldn't really be shared with passenger trains ever, like the Harbor Sub or Alameda Corridor.