Eugene van der Pijll
In steam is it allowed to trace a route that loops through a city twice. It seems clear that no value would be scored for the second pass through, but can the path be traced. And I suppose it should also be asked whether the second pass counts as one city toward the train length limit. (I would think yes.)
This is not allowed. A train can only run through a city once (except for Amsterdam, which has two separate stations in yellow and green), as explained on page 8 of the revised manual here on BGG.
If you have more than one train, they can visit the same city, and its revenue counts for both trains.
(And you cannot run through the same city twice in Steam either.)