Coaster and Surfliner do operate over Miramar Hill, which has a grade but isn't very long. Automatically blended brakes should work fine there especially considering the short trains usually used. Metrolink & Surfliners north of LA can also see some grades, though not very steep and not very long, in Soledad Canyon and west of Chatsworth - speed limited more by curves than grades, I think, and again trains are not very long. Otherwise, the lines in California are pretty flat. Now if we had something going over Tehachapi, or Cal Amtrak (not long distance) going over Cuesta or Donner, a little more braking flexibility might be helpful.

Another thought for running from cab cars: in addition to the control wiring issue, with the locomotive pushing, wouldn't you get noticeable slack action just switching to dynamic without some braking throughout the train? Going from fully bunched (pushing) to fully stretched (braking)? Always having blended braking should help with that. As noted by others, a lot of these trains are just accelerating and braking, not cruising much, due to the station stops.