The more routes into a city the faster it can grow. I tend to make a hub city as my county seat. Each city has route into the hub. That is 6 connections on a small map. Then, make sure you deliver the freight it needs. Have 3-4 bus routes feeding your people moving stations.
Just a note here, I have 6 hubs in the hub city that act as main stops. I try to keep these on the outside edge of the cities. A hub will consist of a large bus and tram station. I used to use just Tram Stations, but every game had to be restarted at some point due to traffic lockup at station. With the bus/tram station setup I have not had this issue.
You want that main hub to have 75% or higher people using your transport. By about 1930 ish, you should be sitting at about 1000-1100 pop. I have no idea how people get to 1000 by 1900, I know I cannot do it, the best I have been able to do was about 800.