Let's consider then the difference between a car and train based commute for people who commute to a downtown business center such as San Francisco. A major suburb or satellite city lies 50 miles from the main city center, and a small city lies 100 miles from the main city center. Let's assume that the driver can average 60 mph on the main freeway and that the train can average 150 mph (this is a liberal estimate, considering that much of the time would be spent within 50 miles of the city center, where speeds are typically slower.) I'll compare those that drive to those that use a combination of train and walking or biking. I'll address the hybrid case after.
The green areas represent the places that people can live in order to commute to downtown in the main city. In the car case they can live anywhere within an hour's drive where roads exist. The drivers are able to sprawl fifty miles from the city center and sprawl ten to twenty miles in the suburb or satellite city (depending how close they are to a freeway) and still make it to work in around an hour or less (assuming there isn't horrible traffic, which there often is.)
Now examine the combination of high speed rail and regional rail, and subways or light rail downtown. Outside the downtown area, the rail riders are limited to small green population centers within a maximum 5 miles of a train station, whether it's high speed rail or a regional rail station that connects to high speed rail. However, the rail users are also permitted to live within 5 miles of the small city that is a 100 miles from the main city's center. The hundred mile commute from the small city will reasonably take around an hour to walk about 10 minutes to and from each station and take the 40 minute ride. Although drivers can't drive from the small city in an hour, it's hard to say that living within 5 miles of the small city's train station is sprawl, especially since the majority will need to live within 1 mile of the station to walk. Moreover, since all the rail users walk or bike to a station in their neighborhood to reach the train, it's certain that they live more "locally" than those who drive directly from their homes to the main city center without walking any local streets.
The high speed rail critic will now site the hybrid case as the reason that high speed rail causes sprawl. Imagine if a commuter used a car to access a high speed rail station to get to work? Where else could they live in the rail diagram above? They could live reasonably within ten miles of the high speed rail station in the suburb or satellite city by doing the following: 1) A 10 minute drive to the station (if they had uncongested highways); 2) Plus 10 minutes to park and catch the train (realistic given that the they'd have to drive into a parking garage, take the stairs, walk to the platform, and wait for the train, which is harder to time when driving than walking; 3) Plus 20 minutes on the train to go 50 miles; 4) About 10 minutes to walk to their final destination. Thus, it takes a minimum 50 minutes if all goes well to get to work if living 10 miles from the suburban station, which is how long it takes to simply drive directly to work if they live on the closer side of the suburb. Living in the small city 100 miles away and driving to the station would take over an hour if you add 20 minutes to the above calculation to travel the extra 50 miles. The hybrid scenario greatly increases the chance of unexpected delays, since commuters are subject to the incidents of both driving on highways and the train. Thus, the sprawl added by high speed rail when people drive to the station looks like this:
That yellow area on the far side of the suburb or satellite city is the only contribution to sprawl that high speed rail can make. You can live an extra 5 miles away from the city center and keep your commute to about an hour. It wouldn't be realistic to drive more than 10 miles to the regional rail station in the suburb or satellite city, nor to drive more than 5 miles in the small city, to access the high speed rail network. The time penalties of driving to a station, parking in a garage, and getting to platform are significant.
My conclusion is that high speed rail can't do much for sprawl, except for the outliers willing to commute over an hour, and remember that a 30 minute commute is average in most places. The sprawl that is enabled in the the yellow area above is still a difficult commute given that two different motorized modes of transit must be relied upon. I bet it would be hard to drive more than 10 miles to that suburban high speed rail station and make it to work reliably in an hour.
On the other hand, high speed rail can do a lot for living locally. It gives regional rail and light rail a backbone to connect to, so that anyone living within walking or biking distance of any rail station can commute to the main city's downtown in a reasonable amount of time. It's a lot easier to time a regional rail to high speed rail transfer than a car to high speed rail transfer, because the schedules are designed for it. Also, all that time on trains is time that can be spent working, which allows some commuters to get work done on board instead of at the office. As long as all the trains in the network give everyone a seat to work, rest, or sleep in, the time on the train is productive (metro and light-rail should never be standing room only.) The productivity of train commuting can actually give people more time at home or hanging out in their neighborhood. And the driver will inevitably bypass their local business district in favor of something convenient near the freeway in order to do errands or spend leisure time.