This is so easy achievement, just start a game against 1 Marg Loughbot and play. I had ticker Murmansk-Kobenhavn so I decided to builds routes Honningsvag-Kirkenes and Arhus-Kobenhavn. You shouldn't forget to win, so then do some other destination tickets.
24 train way is: Honninsvag- Kirkenes- Rovaniemi- Tornio- Boden- Umea- Sundsvall- Orebro- Goteborg- Kobenhavn (or Alborg)- Arhus
or you can Rovaniemi-Sundsvall take in cities (Tornio-) Oulu- Vaasa
There are maybe more ways, I am not sure...