Ah.... commuter rail is very near and dear to me. I use it to get to/from work daily (and have to catch my train in a few minutes, so I'll try to be brief!).
With the exception of Indiana and Virginia, the other "15" states are "BLUE" states(Democratic)
Commuter rail or light rail is also fairly established in Dallas and the Palm Beach-Miami corridor. It is either under consideration, starting up or expanding in places like ABQ, IAH, BNA, AUS, and DEN.
If I'm not mistaken, more than a few of those are "RED" states, so please don't try too hard to turn this into a political football. It's really just a factor of when those cities expanded into major metro areas.
Chicago and the two coasts are much older metro areas, and are heavily invested with commuter rail because their suburban areas were built around the rail lines. Look at growth maps of metro NYC or Chicago, and it's pretty clear that the oldest towns are within about 5 miles of the rail arterial in/out of the downtown area.
Cities which built up during the interstate era (1955 forward) have growth patterns around the freeways, and don't have the same rail arterials with which to leverage, which makes it a little difficult to introduce rail transit. Either the rail lines are in industrial areas and away from the residential areas (which is the case with the Fort Worth to Dallas line currently in use) or the lines bypass the populated areas because of all the activists who insisted that freight trains and houses don't mix very well at 2am, and the lines needed to be rerouted.
Even in some of the new growth areas of established cities like Chicago are having a hard time expanding commuter rail and light rail for the same reasons newer cities do. Plus, real estate is not exactly cheap, and you need a lot of right-of-way to make any rail transit project work.
The Republican Congress(whether in "power" or not, has vehemently opposed ANYTHING to do with Rail, (except freight), for as long as I can remember !!
Hold on, Bears... Passenger railroads saw the largest decline between 1960 and 1970, which was during Democratic administrations. Amtrak was formed during Nixon's tenure as a way to save passenger rail and try to keep the freight railroads from certain insolvency (a little too late to save Penn Central).
Clinton didn't exactly do a whole lot to build up passenger rail, either. In fact, didn't Amtrak run up some of its biggest deficits during his administration?