That's interesting that California gasoline consumption has not gone up as much as the rest of the USA. However I think you are jumping to conclusions to say that this is because of rail. We would need to see more evidence to figure out what is the cause (probably more than one thing).

California has considerably higher gas prices than the rest of the country, for example. Yesterday's price report still has California above $3, while the US average has been lower than $3 for several weeks. Maybe these high prices have played a role.

Another possible difference is increasing urbanization of the state. As the demographics have shifted from republican to democrat we might have seen an increase in the percentage of people who live in or near cities, decreasing driving distances.

The point is that in a state as large, populous, and geographically and demographically diverse as California, you will probably not find one factor that drives gasoline usage. Whether railroad travel is playing a significant role is not possible to determine from the evidence given.

Also, you can get those good gas-consumption numbers by simply importing lots of poor people who only get around by bus or bicycle, and who have large familes thus raising the capita in per-capita. Also, Chinese and Indian etc immigrants here, while middle class, are typically single-car families with lots of kids, again the good numbers are the result of tons of poeple coming in.
I sometimes wonder if California's per capita gasoline consumption is limited by its gridlock.  I mean, Jevons advocates ... come here and try to rack up that greater consumption.

(I understand that a gridlocked car gets lousy mileage, but I think the congestion keeps a greater number of cars from even making the attempt.)