Today Caltrain can't run more trains because diesel locomotives are too expensive and slow, and the current route has no passing tracks which makes scheduling h

Today Caltrain can't run more trains because diesel locomotives are too expensive and slow, and the current route has no passing tracks which makes scheduling hard to work in with express trains, which are needed to stay competitive to commuters. When they move to more modern EMU trains over the next couple years, they will be able run with 10 minute headways, potentially down to 6 minutes if you include the blended service with the CAHSR trains. But that only solves the waiting-for-a-train problem. Regional train line still have trouble keeping up with expressways because of the time needed slow down, wait, and then accelerate up to speed at each station. Lighter rolling stock can make this a little bit better, but not dramatically; BART has always used relatively light EMUs and it still can only average around 55 km/h. Even the most fanciful of Caltrain upgrades would have a top speed of 180 km/h, which means they'd really be lucky to average 100 km/h on a train making every stop between San Jose and San Fransisco. Express trains would be slightly better (perhaps using the HSR trains to do SJ<->SF nonstop in ~30 minutes) but that could only cover a subset of trips. For most trips, if you can avoid terrible traffic, driving will remain faster.