I commonly play on hard and find it is really tooeasy to make money – even with trains I usually make lines that usually display profits o 1-3 millions that connect several cities and have several trains on them – to the point that my only challenge is trying to reach 1 billion before year 2000.
I usually start with cargo lines and inter-city busses for the first 10-20 years. Then I am usually switching to trains because I like them most even though I am not sure if they are economically better. Of course you have to feed your railways lines correctly with intra-city busses (which usually have a slight loss in the order of 0-50K).
Yes, the game would benefit from some balancing between busses/trams and trains: busses are usually cheaper in total cost per passenger and km (considerng also depreciation, loading capacities and speed), especially in the beginning. Trains also require a costlier infrastrucutre.
On the other hand, train usually have a higher speed and therefore allow passengers to travel farther in the 20 minutes line (providing you give them the possibility to do so) and thus you can increase the fares they pay (which, I guess, only depends on distance). Considering that, I am not sure that inter-city busses would be economically superior than trains, provided you connect more than one city and allow passengers to travel farther.
All considered, I think the game is fairly well made given its complexity and I am certainly glad I bought it.