You know that the smallest ever subway town is in Austria? Its in a village even. But Lausanne is the smallest town in the world with a subway. My cities in Train Fever are big enough to have 4 or 5 underground stations at least. In the end, these cities will grow into each other and then its even more important.
This must be implemented into this game:
1. Avenues
2. One way roads
3. Roundabouts
4. Traffic Lights
5. Light-rail track system. Smaller stations for Railbuss system etc.
Then this game will be perfect. Subway is only an option later if the game will get underground view.