So I figured out some stats for the transportation hub buildings. These are your primary source of land-based foreign-connected commerce.
Caravanserai is the first hub. It is basically a reskin of the
Paved Roads. However, I assigned it to a different tech:
Horse Breeding, because I thought it could use help and I like to mix military and non-military tricks on the same tech. I emphatically do not want it assigned to
Trade, as Trade has by far the most tricks with 10.5 (which will be cut to 8.5 once Silk Road is reassigned and Silk Farm rolled back into Plantation), so it really doesn't need any more.
Caravanserai also has no resource requirement and has its maximum commerce from foreign trade capped at +15. I thought +25 is way too much for a building this early.
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Railway Station upgrades Caravanserai at
Railroad. It increases the maximum commerce bonus from foreign trade to +20. It also provides some domestic commerce, 1/3 cities up to +10. Finally, the Railway Station provides +10% production as it makes it easier to ship raw materials around.
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The last trade hub is the
Transporter. It comes available at
Teleportation. It is an upgrade of both the Railway Station and the
Personal Rapid Train, as it can transport people and material. So it provides a lot of bonuses from both. I figured this will satisfy my need to have buildings dovetail when possible, but the Airport line will stay separate.
The Transporter provides +3 commerce from a capital connection and increases the maximum commerce to +30 from domestic connections and +30 from foreign. I almost lifted the limit completely, but I thought that might be too much. It increases the production bonus to +20%.
Since the Transporter upgrades Personal Rapid Train, it also provides +2 happiness (increased from Personal Rapid Train's +1) and +4 health, +0.1 per citizen (PRT provides +0.1 health/citizen, +1 health generally, +1 from Oil and +2 from Green, so Teleporter is equal to PRT's maximum bonus).
Finally, since the Transporter can teleport things, I added +20% spaceship production (like the Space Elevator, it helps get stuff into orbit faster) and can airlift 2 units per turn. It's a powerful building, but it comes very late in the game.
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