Blogger Atrios asks today if the transit part of T-Rex can really make it possible for people to have one less car outside the central city. The answer is yes. There is a historical precedent, nationally and in Denver. It was called the "street car suburb." Cinderella City, in Englewood, is a contemporary example of the same concept.
The other benefit, which he doesn't address, but which is likely to be even more of an impact, is that transit can reduce how much you drive with the cars that you do have. Most people who take transit to work do what I did when I lived in Washington D.C., drive a mile or two to a local rail station, and take the rest of the trip via rail instead of a car. Park and ride, and the related kiss and ride concept (your loved one drops you off with a kiss at the station), rather than walk and ride, is where most of the driving miles avoided come from in an intracity rail system.
In addition to providing a way for suburbanites to get into the city, the T-Rex line will also provide a way for city residents to get to the Denver Tech Center and the Park Meadows mall in the suburbs. Good rail systems need not just sources, but destinations.