Thanks for the correction; I got the sentence inside out. Yes, electric Caltrain is practically zero localized emissions. And yes, regen will keep brake dust emissions to a minimum. Reentrained dust (raised from the trackbed as trains go by) will probably be more in mass terms.

As for regen - it's in use now. AFAIK, the Caltrain locomotives, like most US passenger diesels since the 1980s, use blended braking. In normal use, that relies on dynamic braking until near-stopped, at which point it automatically shifts to friction brake. For high-rate or emergency braking, of course, friction brakes are used.

For car people, the same setup has been in use since at least the first round of modern EVs and hybrids in the late 1990s. All of the 2000ish EVs had it; it was the only way to get more than a few 10s of miles out of a battery at the time. Honda EVs in our motor pool around 2000 were rated at 120 miles/charge, but only got near that if running around town; my trips 20 miles down the freeway and back burned 1/2-2/3 charge at speed with the a/c running. The first Prius had fully-blended braking (though they didn't get it right, for feel, until the Gen 3). Honda's Insight had a simpler system that ended up working much the same way - light braking was just regen, heavier brought in the discs. For hybrids, obviously, regen recharges the battery "for free" to power electric-boosting a gas engine that's really too small for the car. Note: those first Honda hybrids really did get close to 70mpg. Some of those were in our pool too, and I got an indicated 65mpg for a 250-mile round trip, in stop and go traffic for practically the entire trip (these days, more trains are available for that run and would be used).