Although the trains have quite high costs, if they run properly, they also give you the most revenue. In the early years, one line can give you easy 100.000 – 600.000 revenue. If you compare that with bus lines, you can almost say they are insignificant. In the modern times, some of my train lines give me 6 million in good years. And I run 4 passenger lines, so sometimes I have 18 Million income per year.
But some hints: Don’t run cargo trains in the beginning, a factory has to have about 3-5 cities as connected markets in order to grow to dimensions where it makes sense to use trains.