Rail service (passenger or freight), and for that matter transit service, does not in my observation produce any long-term, and seldom much short-term, improvement in nearby traffic conditions. It does, however, add capacity to the corridor, and may divert some trips making space for others that wanted to happen but couldn't because of the congestion.

Case in point: BART. It was supposed to free up traffic on the Bay Bridge -- NOT. It does, however, make it possible for a lot more people to get to SF, which made the massive downtown development of the 1970s-90s and probably a lot of East Bay development possible. The Bridge was at capacity in the late 1960s; it still is, and will remain so as long as it's feasible for people to drive. Extrapolate that to other corridors if you want to.

However, a decently functioning HSR system can easily beat up competitive air traffic on runs of 400 miles or less (ummm ... SF/LA?) for business (single person) trips. A net running time of 3-4 hours is about the same as what it really takes to fly (1 hr on the plane + 2-3 or more at and accessing airports) and 1/2-3/4 of driving time assuming no significant messes. Those who have a group (a family?) going may still fly on low special fares, with fewer options than they do now, or will just drive like they would now. Those with little money (HSR won't be cheap -- probably business-trip air competitive) will take the bus as they do now. Amtrak as we know it (basically business-class bus) would go away because it would not be able to compete with HSR+local transit or rental cars assuming stops (for the local run) every 75-100 miles. On the plus side, the rental car folk would have to move out of the airports (captive market) into the real world (real-time competition).