Thank you for your tip. Though I am not going to implement as you told
me, I appreciate the approach to use a class Map.

My thought is following:

class City without any references to the connections.

class Connection
    @city1, @city2
end

class Map
    map = Hash.new     #  key == city
                       #  value == Array.new which contains the
connections referring to that city.
    

What do you think about this?

draq