Right, I used to take that a lot, because from the Iron Horse in Dublin south, the Alamo Canal Trail/Centennial Trail was paved all the way to Arroyo Mocho, so you could connect to West Las Positas, or cross the bridge and the Arroyo Mocho Trail was paved up to Hopyard so you could get all the way from Dublin to Hopyard just south of West Las Positas on paved trail.
Then the floods of 2017 hit and ruined a lot of pavement. Now stretches of the trail between the water treatment plant and Arroyo Mocho are dirt/gravel, and the Arroyo Mocho trail between the Centennial Trail and Hopyard is dirt/gravel once again (it was only paved for a few years before the flood). Post-floods, it is mostly easier for me to go under BART unless I'm not on my road bike, although getting on at Val Vista works. I've even used Johnson Drive to get on the trail and go under 580, but that has its own dangers and will be even crazier when Costco comes in.