Which states subsidize trains and why would make an interesting study. It might start something like this:
Some states pretty clearly subsidize Amtrak trains and/or their own commuter rail networks because they have a lot of people living near each other and trying to go to the same places. Mass, Conn, NY, NJ, Penna, Md, VA, IL, CA, UT, CO and some others. (Some of those states may not spend any money on operating subsidies for intercity passenger rail, but Mass owns part of the NEC and at least five Amtrak stations, CT does something similar, Utah's commuter rail also works as a 95-mile intercity train as does Indiana's South Shore Line, etc., etc.) Those states pretty clearly have situations where spending money that directly or indirectly supports Amtrak service is a practical, cost-effective way to deal with local transportation needs and where trains collectively (Amtrak and other) have a significant market share in either commuter or intercity markets, or in both.
Then there's Vermont and Maine, both subsidizing trains in pretty thinly populated areas, but on two different models. WHY? Maine's basically running a commuter-and-daytripper service that is supposed to get more people to live in Maine and work elsewhere (like the South Shore Line) or to visit Maine and spend money there. Vermont's two daily trains seem to be aimed mostly at encouraging tourism from greater NYC, with local passengers as a bonus and local residents leaving the state on business or pleasure as a side effect. Bringing in tourists by train seems to fit well with Vermont's efforts to keep small towns (and specifically their centers or villages) busy and attractive, so that most of the stations in VT have a quaint B&B, a couple restaurants, an ice-cream parlor, and a nice walk within walking distance of the station. VT might not just be a little train crazy, they might also have more railroad-passenger-friendly small towns than anyone else.
Somewhere in the middle are Missouri, NC, Oregon, Washington, Oregon, probably others I'm forgetting that subsidize some Amtrak trains that serve scattered small cities and towns, bigger than in VT but spread across thinly populated areas. WHY? We could rearrange the list. Some states seem to go in two different categories depending which trains I think about. For example, the Illinois corridors would fit here except that they have Chicago at one end. Where do we put Oklahoma? Never mind. The point is that these states subsidize a few trains in places that are not obvious passenger-train markets. Do these trains exist because railfans and local boosters who think a train will bring development are strong in those states, or because there is something about the distribution of the population, the habits of the people, etc., that makes trains look, and maybe be, practical?
Why does anyone anywhere outside roadless areas that happen to have a railroad think a one-a-day train is useful enough to subsidize? I know Amtrak has the LDs because they inherited a network that had already lost most of its branchlines and local trains, and then they simplified it, and the LD trains are emblematic, popular with people who write to congress, useful to certain tourism markets, etc., but why would anyone want to go from 0 trains to 1 train in a place where people have obviously been getting to and from some other way? I guess that's a question for this whole thread: There's restoring a favorite route because we're fans and we want to ride it. Fine. This is a railfan forum and we like that stuff. No problem. Fun. But from a practical standpoint, why does someone in Duluth who isn't a semi-pro railfan like most of us think one train a day from Mpls-StP would be worth all the negotiation, station-construction, etc., etc.? Or one-a-day to Pueblo and Colo Spgs, or across South Dakota, or up into Burlington, to the Quad Cities, or wherever? I just don't see the practical argument, even though they sound fun. To me, you need at least a morning-and-evening setup to make any kind of middle-distance new train make sense, and something like the Downeaster schedule seems much more likely to actually become a significant part of anyone's transportation options. (We can debate how well or poorly the DE does, but I suspect everyone would agree it does more good for its subsidy than spending the same amount restoring the Gull would have (train to NB and NS, right?))
I don't think we need to spend time wondering why Wyoming, the Dakotas, Kansas, Arizona, Nevada, and Idaho don't subsidize any trains, or why Maine doesn't have 4 a day on the old CP line or Michigan doesn't have any in the Upper Peninsula or whatever: either there aren't enough people or the population centers are too far apart, and there's not much traffic on the roads.