I think we understand that there is such a thing as a dual-power locomotive that can run as either a diesel or as an electric. Unfortunately, they're rare and expensive and complicated and hard to maintain - would make even the F125s look like a bargain. And the most recent ones built in the US (for NJT) are significantly underpowered on diesel compared to their electric specs, so beyond the wire they are much slower and/or must have smaller trains.

Absent recent published studies that look at realistic demand WITHOUT HSR (like, perhaps, the ones mentioned at the beginning of this thread that are still in preparation), I still question whether Metrolink has enough traffic demand on *any* of its lines to be in Caltrain's situation. Yes, perhaps, between Burbank and Anaheim (or Santa Ana or Irvine, or ... we don't know yet). Caltrain has an existential problem - it can't meet demand using diesel-hauled trains, and needs MU cars. EMUs are the best choice for that purpose. Where on Metrolink's system is that likely to occur over the next 20 years or so? We'll know more, presumably, after the studies are done. In the meantime, the map is interesting to look at.

And remember, an emission analysis for Caltrain is mostly comparing electrics against Tier 0 or 1 locomotives (some of Caltrain's almost qualify for antique status, and few if any are as good as Tier 3), while Metrolink has newer stuff including (as the F125s get patched up enough for service) Tier 4. While Tier 4 is not zero local emission, it's low enough that the cost of going electric (as Bob has noted) results in really poor cost-effectiveness as an emission control measure. Electrification of Metrolink has to be justified on other grounds.