They must grow from existing cities as far as I know. It’s not like Atrain where you can just deliver materials to an area and they’ll start plopping down houses.
you can surround and squeeze existing cities with rail tracks. Once all space is used, cities will start to spawn outside of the rail tracks, where ever there is a road.
To make a city “appear” somewhere, just put some roads there. you will have to delete all other nearby roads, however, or else the city will appear there