Several others have commented on not necessarily seeing the elements that are usually associated with film noir in the opening scene and I think, for the most part, I do agree. Truly a dynamic opening and, in some respects, a beautiful and optimistic salute to human ingenuity where the two conductors know their train, their craft, and each other so well that communication can be conducted with whistles and hand movements. However, I would have to say that the train passing through the tunnel (and the light at the end) or, even, the train passing another train at high speeds do convey a darker and potentially looming tone to their arrival at the station.
Yes, very good use of shadow, especially with the clever use of shadow over the wanted poster. I was curious if the song that the children sing is an actual rhyme sung by German children in the past. I found several possible links to potentially related German songs and games in "Der Fuchs Geht" and "Wer hat Angst vorm Schwarzen Mann?" Not necessarily completely relevant, but children's songs have always had an omnious streak, such as "Ring Around the Rosie" (which I just read may not have anything to do with an outbreak of the plague) or "It's Raining, It's Pouring" (which could relate to a head-injury). I agree with others that have mentioned the juxapostion of innocence and danger, it seems very interesting that children are singing such a song and may, in fact, be much more aware of the omnious setting within the film than the adults acknowledge.