The DLC with three new cities will be available on the 14th of September.
Follow the link to take a look at the DLC:
https://store.steampowered.com/app/1397250/My_Train_Arrives__City_pack/
The first DLC
Loading news and updates…
Source: Steam Community (opens in a new tab)
https://store.steampowered.com/app/1397250/My_Train_Arrives__City_pack/