I regularly add tram lines to my hub cities, and I currently got in a city a population of 1500 in only 80 years (1850>1930), on hard difficulty. The city has 2 train lines, 5 tram lines and 1 truck line for goods.
I think that the secret is having an high % of population using your lines in the city. In the city that I mentioned I have 90%, but I think that a 70/80% is enough to stimulate city growth. I see that cities with 50% or less have almost no growth. Of course, you must also have the goods need completely fulfilled.
To make the population use you service (and to have a good profit too), your lines must connect strategic points (using the zone map).
What a pity that in 1930 my game is already almost unplayable due to the high lag, there is too much people moving around. When a lot of cars will appear in 1940/50, my map will completely freeze….