This. People tend to grossly underestimate the expense and effort to build new rail tracks in an industrialized country. Most land is owned by someone already, there are environmental concerns, other (rail-)roads and generally obstacles must be crossed, and -worst of all- you have to repeat the process basically for every single town you want to connect.
Side remark: The same holds for roads, of course, with the notable caveat that roads form a flexible network automatically. Planes are just superlinear in their flexibility: If you have n airports and add one more, you get n new connections.
A a route that serves a few hundred passengers a day is also potentially profitable for an airline while a train would likely not be unless there's existing infrastructure that can be leveraged. Also, if nobody needs that route to be served any more, just send the plane elsewhere.
Edit: I'm an idiot and missed that these are 19 seat planes. No, a train that only serves 19 people a day, one way, has a zero percent chance of being profitable, and I doubt it even makes environmental sense if you have to lay 70 miles of track to support it.
For 19 people on a 100-mile route, they may as well schedule a bus.
(For example, my parents would take a direct bus a similar distance to Heathrow or several other airports. It saves parking costs and driving while tired.)
Side remark: The same holds for roads, of course, with the notable caveat that roads form a flexible network automatically. Planes are just superlinear in their flexibility: If you have n airports and add one more, you get n new connections.