Growing other cities will take too long on the 4 and 8k citybuilders and isn't needed on any of the CBs.
Best is to start with connecting a few cities to your town by train. Distance should be medium/long, so you'll get some nice money from it too. In temperate 4 and 8k, don't use electric, go for the Floss or the SH'125 if available, the normal rail is cheaper, as are the diesel trains. The Floss is only slightly slower than the best available electric and the SH is faster when it becomes available.
In 25k you can go monorail or use planes to get money (between 2 cities for good money, from city to your town for faster start of town growth, but less money).
2-3 cities should start you up nice if you build your stations in a good spot or feed them with busses and mail trucks from the cities.
Most important thing for fast growth in citybuilding besides meeting all requirements, is the rating of the local authority.
To get and keep this high, avoid killing too many trees with roads, tracks and stations.
To boost rating you can plant some trees (but I'd advise against it). This won't help limitless, only if rating is low and if you'll have to build over them again, rating will drop drastically again. Best thing to get the rating to maximum is to have some busses drive between a few stations in your town.
Each month players' ratings automatically change:
Rating goes up by 5 if it is less than 200.
Rating goes up by 12 for each station that has transferred cargo in the last 50 days.
Rating goes down by 15 for each station that has not transferred cargo in the last 50 days.
wiki.openttd.org/Game_mechanics#Local_authority_rating 
