a. Suburban-to-city center commuter trains stopping every two miles or so, vacuuming up all the people from the suburbia that want to get to the city without the hassle of endless traffic jams. These need a certain density of the suburbs, so that the stops make sense and the train is "just full", but not absurdly crammed.
b. High speed connections between metropolitan areas, like Madrid-Barcelona, Tokyo-Kyoto, Köln-Frankfurt etc. Usually, at least a million people must live on either end of the connection.
One of the problems is that these two services need separate tracks. Suburban commuter trains are much slower on average than high speed trains and they cannot share the same track, unless you want to risk chaos every day.
Unfortunately, few densely inhabited places in the U.S. have space left for so many tracks, especially in the wider city centers. A potential renaissance of rail travel in the U.S. would likely require underground tunnels. Which is possible (see London's Crosslink project), but not exactly cheap.