In 2009, Caltrain projected 72 thousand boardings by 2030, a rate of annual increase of 2.3%. These projections are obviously extremely important, since capacity increases need to be planned well in advance.
Indeed, as was predicted by essentially all rational observers at the time, the ridership has exploded far in excess of that "extremely conservative" schedule, Ridership survey reports are available here. Here's the annual ridership:
At the rate ridership has been increasing it would hit 72 thousand not in 2030, but rather in 2019:
A key thing, though, is that ridership will not increase at this rate. There simply is not the capacity. At some point trains get so full that the experience of riding on the train is worse for a sufficient number of passengers than the experience of driving, or the experience of working from home, or the experience of moving closer to work, or the experience of changing jobs closer to home, or the experience of moving to Portland. There's a breaking point.
Here's a photo I took on a recent commute on one of the 60-something days when the San Francisco had a weekday evening home game. It was like this all the way from Mountain View to 22nd Street, where I somehow managed to fetch my bike and get off the train:
But this argument applies also to past data. The rate of increase would have been even greater had Caltrain had the capacity to support it. Even I, with an exceptionally strong anti-car bias, have been pushed to close to the limit by commutes like this. My alternative isn't driving, but changing life circumstances in other ways to avoid the problem. If Caltrain doubled its capacity today, it wouldn't need to wait for 2030 or even 2019 to hit that 73 thousand number: it would be there by next year.
Instead, it responds to record ridership numbers by slowing the trains. With "deferred maintenance", with too many riders boarding trains purchased with too few entry points into too few cars, the allocated 2 minutes per stop isn't enough.
To a large degree, Caltrain rules its own future. More service = more riders. Status quo = poor service = limited ridership and reduced quality of life in the Bay area.