Building Railroads

How does the production bonus for railroads work on archipelago maps? Specifically on single city islands.

Jeremitus I said:

How does the production bonus for railroads work on archipelago maps? Specifically on single city islands.

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Essentially the part of the trade route that runs between harbours is automatically treated as railroad once the tech is discovered (might take an extra turn before you can see the effect). If there is then a connection that is all railroad to the capital, you get the bonus.