The Grey Lokomotive is the achievement to unlock extreme mode for Berlin, the map the achievement has to be played on to get. To get it you need to deliver 1000 passengers in Berlin using only one locomotive per line.
Since the achievement renders half of the budget increase useless, because you can only have one locomotive per line. You should never get the tunnel upgrade, but have two lines running to connect the two the sides of the river. I would recommend to have two lines (including the connector) on the bottom third, three on the top, and one on the left side, but it should have the majority of it in the top.