I believe Rockville Station is the next station west of Gillespie’s Store in Vacaville. At least, I haven’t found references to any other. [Well, there is a plaque at a nearby winery that claims to have been a Pony Express stop, but the location and lack of other evidence makes me question whether that’s true.] You can read more about the Rockville monument here.
The Rockville site, like some others in California, is not auspicious. It sits on busy corner in front of a closed-down bar. I can say, though, that there are other attractions nearby. There is a Valley Cafe right next door (great for breakfast), La Barrista across the street (good sandwiches). And a mile or so away is Rockville Regional Park , which has some great mountain biking trails.
I doubt make Pony Express Bikepacking Trail riders will make it this far west, and that the few who do will be looking for big climbs and descents and rock gardens and the like. But there you have it, for what it’s worth.