Many good ideas around here.
I wouldn't like to see a railway district because it seems to narrow a concept to spend a district on. Airports suffer from the same flaw atrocity already but they are somewhat justified because air units are a new special kind of unit then.
The above IZ building is a sensible approach and similar to what I was thinking when suggesting the Agricultural District.
However internal trade routes are already the overwhelmingly predominant means to increase production in select cities and I see no pressing reason to improve them even further. The mutual exclusivity with factories/power plants is a non-choice because there is little incentive to build overlapping factories/power plants since the stacking is gone and the new building adds production to the host city much like a factory/power plant would.
Coming from real life and previous civ versions there is three primary aspects to railroads that could be added:
- the logistical aspect regarding easier troop movement in home territory
- the industrial aspect regarding the transportation of large amounts of material to factories
- the commercial aspect regarding increased consumer traffic and transportation of retail goods
The logistical aspect
I believe that Civ VI got this one covered already. Upon entry into the industrial era, existing roads give a discount to movement point costs. This probably not only represents the improvement of the road network per se but also the utilisation of new means of transport like railways.
While it is to some extent true that movement by rail should ignore movement points and treat all units equally, this is not strictly necessary from a gaming perspective. If you don't want your tanks arriving faster than your artillery, the tanks need to slow down.
Adding new movement options could also conflict with the modern era road upgrade. Rails would be superficial if they were worse than modern roads or modern roads would become pointless beside a railway network.
Therefore I don't think, that railroads should tangle with the movement aspect of the game.
The industrial aspect
Production is in high demand so bonus gears would be on everyone's priority. In order for railroads to be a meaningful choice, it wouldneed to come at some cost or force otherwise meaningful decisions.
Employing a building that excludes something else which is normally desirable would work. But as said above, excluding factory/power plant doesn't work like that.
Making it a district doesn't work either because there is already a district which promotes industry. If anything, it would need to be a side effect like with encampment buildings.
In general though, adding industry is a sensible choice.
The commercial aspect
With better means of transport comes increased trade. Making railways a means of making money seems natural.
It also wouldn't step on the toes of any other concept currently in place. One could argue that lategame trade-enhancing policies alteady take into account the utilisation of new means of transport but that conclusion isn't necessary. Most are spun in a way that political development is the key means of increased trade.
So placing a building into the commercial hub could make sense but it would need another mechanism than just adding a flat amount of gold like the other buildings do. I also have reservations against adding more utility to the commercial hub because it already is on everyone's to-do list. For the same reason, adding a new trader slot via a commercial hub building is out of the question.
The conclusion
Taking into account what I said above, I see three possible avenues to treating railroad in Civ VI:
The Railway Station District(industrial era)
Can only be placed next to the city center
Adjacency bonus +2 production if adjacent to an industrial zone, +2 gold if adjacent to a commercial hub, +1 gold and +1 production per two adjacent districts
Yields: +1 production internal trade, +2 gold international trade
1st building - Main Station (comes with the district)
+1 gold, +1 housing, +1 amenities
2nd building - Freight Trains(Modern Era)
2 gold maintenance
+3 production, +1 production per industrial zone building in the city, +1 production internal trade yield if no industrial zone in the city, +1 production per bonus resource and strategic resource produced in the city if no industrial zone in the city
OR
Commercial Warehouse (same tech as Freight Trains)
+2 gold per commercial hub building in the city, +1 gold per luxury resource produced in the city, +1 trade route capacity if no commercial hub in the city
3rd building - Rapid International Trains (Information Era)
4 gold maintenance
+2 tourism per international trade route, +1 culture per internal trade route
District project: Shipping Effort Provides gold equal to 15% of production and an equal amount of Great Merchant and Great Engineer points
Espionage action: Disrupt Transportation network Nulifies the effect of internal trade for x turns
While I'm not the most fond of making railways a district, I think that this might work out. Placement restrictions force players to plan ahead and possibly hamper themselves early for a later bonus. It reduces dependability on commercial hubs which is a good thing in my opinion.
But you could also do:
Building: Central Railway Terminal
industrial era city center building
6 gold
+1 housing per neighborhood, +1 adjacency bonus to all districts, +1 tourism per wonder, +1 tourism per 3 great works/artifacts, +1 tourism per seaside resort
This one pretty much enhaces all aspects of the city and works best in your core cities to make them even more powerful. Adjacency boni would be doubled via their respective policies which slightly helps them not being overshadowed as much by the building boost policies.
The Trade Post Enhancer
With the advent of railroads, traders get a new option. Instead of regular trade missions, they can be sent to improve trade post.
Eligible targets are only cities with a trade post. The mission has the same duration as a regular trade mission but nets no profit. Instead, the existing trade post will become a railway terminal with the following effects:
If in a foreign city, the tourism bonus from having a trade route to that city increases by +5% (non stackable), spies in that city get an increase to their escape chance and return speed and normal trade routes have their yields increased by 20%.
If in a city state, the trade route effect is as above but you will also earn a one time bonus of influence points equal to the amount of envoys you have in that city state.
If in an own city, the city will get a permanent +2 gold and +1 production, internal trade routes will add a +10% growth modifier and units produced in the city get +1 movement point in their first turn only.
Those are subtle boni but you are not giving up more than the yield of one trade route for the standard duration.