I can only speak for the iOS ecosystem since that's what I've worked in. The mapping part which used to be the most difficult part of any computer program is now pretty easy, in both ecosystems I think. There's a mapping API that you can you. You provide it with log/lat coordinates and tell it was sort of icon to draw. Easy as pie. Hosting should be relatively cheap. Actually it could be free but then the update experience would kind of suck. For the free hosting you could embed the set of coordinates for the stations in the app itself. For iOS you don't pay for hosting of the app, Apple does, and they take a 30% cut for that (and other things they do for you). If you were to have an automatic update of the set of coordinates then you could host it as a compressed json file for example and have the app opportunistically check for a new version of the file every day or so. Hosting a small text file isn't very expensive, pennies or a few dollars a month depending on how many installs, the size of the file, and the rate of updates.
Now by your questions I assume you aren't a programmer. That's fine, in fact, it excites me when people who aren't programmers want to learn because of some project like this. You can learn iOS development from a bunch of free sources but a lot of people like the Stanford iOS development course for beginners on iTunes U and other places I think like Khan Academy. If you own a Mac starting costs are going to be zero. You don't even need an iOS device, but it would be useful later on in development.
You might also want to read up on the economics of app development as they have changed recently a lot. Android has never really been a good place to make money for paid apps. iOS has been getting worse for the smaller developers but it still can happen. And if you are really solving a problem that you want solved and you aren't relying on this for your income then I would say who cares that you aren't making anything. Marco Arment is as good of a person to start reading as any. You can start with these posts:
https://marco.org/2014/07/28/app-rothttps://marco.org/2015/01/15/overcast-sales-numbersHe's a successful app developer, which isn't the norm, but writes a lot about what works and what doesn't in the app store. If I were jumping into this business at this point in the app store's history I would starting by reading all of his posts on the app store and whatever he's linked to. There's a pretty good community of indy app developers that write honestly about the experience. And it isn't all roses.
That said, post-FIRE (which is hopefully soon for me) I plan I finally working on some ideas I wanted built for a long time. The last app I worked on was in 2008 when the app store had just been created. But it was probably a very different experience from that of an indy developer since it was a big company free app, not a money making thing. I've played a bit with the newer APIs and Swift and they look awesome. I can't wait to have the time to spend more time coding.