I think the key word there is "a rail line." Much of the LA to San Diego route is currently single-tracked -- trains traveling in both directions share a single

I think the key word there is "a rail line." Much of the LA to San Diego route is currently single-tracked -- trains traveling in both directions share a single physical track and have to be scheduled so that only one train occupies the track at a time. If your train falls even a few minutes behind schedule (and this seems to happen a lot), it starts missing its slots and ends up spending time stopped on sidings waiting for trains going the opposite direction to clear the track. Now your 3-hour trip turns into a 5 or 6 hour trip. But yeah, if you wanted to start building high-quality inter-city rail in California, LA/San Diego seems like an obvious place to start. They're the 2nd and 8th largest cities in the US and only 120 miles apart. You wouldn't even need "high-speed" rail to make it work. A train that reliably averaged, say, 80 mph could make the trip in 90 minutes, which is a lot faster than driving most of the time. On the other hand, I guess "reliable rail that's not terribly slow" doesn't get people excited like "high-speed rail" does.