I think that the description is not explanatory enough for this achievement. I had to look up on the Steam Community which cities the developers meant, as the map of Switzerland is (at least to me) not clear enough to notice them all in one single glimpse. The cities you want to connect with each other are:
- Genève
or Lausanne (it's a big lake and connecting either city will count)
- Neuchâtel
- Interlaken
- Zurich
- Lugano
- Locarno
- Kreuzlingen
The destination tickets you pick in the beginning ideally have to be somewhere near the route in between these cities. When I was awarded this achievement I was able to start the game with:
- Lausanne to Interlaken
- Neuchâtel to Winterthur
After I completed my destination tickets and connected the lakes, I made sure to spend my remaining trains on the 5-piece tunnel connecting the cities of Chur and Brusio as this is worth 10 points. My railways also gave me the bonus for having the biggest railway network.
My yellow network of railways ended the game with a score of 86 points against my opponent Marg Loughbot who still managed to get 79 points. As a visual aid for those who may need it, this is what my achievement-winning game ended up like:
* if for some reason the image is not present in this solution, just let me know, I can re-upload it! :)