humm irl, railroads, are really different from roads, so I never like the idea that roads just become railroads, what would it mean? visually it looks off, and then how does it translate to gameplay in a meaningful way too?
with railroads, you can carry a lot at a lower energy cost, but it's more expansive to set up (I think? it is more complete and precise infrastructure plus you need the vehicles as well)
it wouldn't make sense to have as many railroads as you have roads on a landscape, railroads aren't used constantly like roads, I mean there is not a vehicle on there all the time, sometimes even kinda rarely
it can give a production boost (easy fast and cheap transportation of large quantities/heavy materials ;; cheap because the way trains are fueled, and they lose less energy, its cheaper to run once its set up i think), a commerce boost (well same thing you can just ship stuff you sell more easily so you might do it instead of not, or cheaper, also since it can be state ran your state can catch some of the money when used for commerce), and irl it's also a big boost for mobility of people since you don't need to use own vehicle to travel (and it can be cheaper as well), and it produces a lot less pollution than highways. So that's where I would think of gameplay translation for railroads, production boost, gold boost, culture boost.. maybe a little boost to a lot of things that have to do with people being able to travel more easily (science culture.. and faith???lol ; I would say science & culture is nice). Let's say you have a sience boost / culture boost to a city connected to another city by railroad provided that the destination city possesses a campus / culture district. If yes, then the other town gets a science / culture boost. Otherwise just a base production / gold boost. I think, in terms of game, it should be worth establishing only as a choice and not "micromanagement" so, it should not be obvious that it is worth it. But irl, it's hard to make a good case for not having trains between two towns if they are pretty close to each other. So perhaps in civ, it can be something costly in time and resources to set up, like you have to do it tile by tile with builder, idk lol (so that you might only do it if you have the builder charges to spend for it)
also roads can't all be highways, highways have a big environmental cost
railroads just need to connect to train stations. Imo, a train station district is weird cause, it's not like a airport it doesn't actually produces that much bothering, train stations are often in city center. there is usually a neighbourhood right around the train station.. it's a urban thing really, it's another advantage of railroads, they can take you right in town cause it doesn't actually use up that much space either (vehicle stays on rails, and you need a lot less vehicle space per people). Well, if that district could have some housing to it as well or something.. or just be a building for city center. It's another thing thats not trivial with the district system, because it can be said of many districts, holy site, theater quarters, etc, many can just be in "city center" irl
well, there are many ways to make it or not make it,
but imo railroads could be interesting, but I don't like to see railroads replace roads. trains are awesome, it's public transportation, rather environment friendly, its really efficient.. a special kind of improvement with its own unique perks
(oh and f*** corporations owning railroads lol, its much better as a public service)