The customs is technically an industry as the others. So you will have workers that are working there, to operate the customs and guarantee the working of it.

Once customs are up and running, tourists and foreign workers will come, and leave when they are done with what they came to do.

So you have indeed a small demand of workers, to generate a larger amount of visitors. ;)

Got it - it just seems that the same amount of people came back from it as went to it.
(they got a dedicated train line from the next city to the customs at the map border)
Perhaps I need to investigate that further.