The SP East end of the bridge is on the Pacific plate (Logan, geographically west, behind the train), the West end is on the American plate (Chittenden, geographically east, behind the photographer). In other words, it crosses the San Andreas Fault (which is also the scar running through the trees on the hill to the right of the locomotive). The granite being quarried at Logan is part of the same granite rock of the Sierra Nevada Mts (the Trinity Alps are also part of the same geological formation, but they were separated by the intrusion of Mt. Shasta and all that.)