It's just a question of scale and playing in the right way.
At the risk perhaps of stating the obvious:
Coal pays well at the start of the game. Take it to a Power Station. You can almost build an entire empire on this alone.
Industries are loosely connected.
If you take Wood to the Sawmill, you also need to take its output of Goods to a town, otherwise the Sawmill will close.
Keep an eye on your natural resources, eventually the Forest will stop producing Wood.
Send Wood from several Forests to one Sawmill. High input = high output.
Similarly, take Iron Ore to a Steel Mill, take the Steel to a Factory, take Goods to a town.
The Factory will also want Livestock and/or Grain.
Concentrating delivery of resources to a small number of regional Factories, Steel Mills, etc. can help you scale.
Similarly, placing stations where they can produce cargo from multiple sources can be beneficial.
Placing stations at Factories, Steel Mills, Sawmills, etc. means they produce a single type of cargo.
You can have two stations at a Factory, Steel Mill, etc. Deposit cargo at one, pick up cargo at the other. As you scale, this will help you manage the number of trains as they are using separate infrastructure.
Passenger rail and mail services have to be fast to make money.
Passengers don't like waiting around. Frequent trains are likely to increase demand.
As you scale, the efficiency of your network becomes critical.
Build as straight as you can. Minimise sharp turns. Trains slow down a lot if they have to turn through 90 degrees within the length of the train.
You don't want trains kept waiting to unload their cargo - the longer they wait, the less you get paid. Build big.
You can build simple and complex track layouts to filter and prioritise faster trains.
Learn how the different Signal types work.
As locos and multiple units age, they become less reliable. You can force regular maintenance by building your track in such a way that the train must pass through the Depot to continue on its journey.
Alternatively, provide an appropriately lengthy siding and place your Depot off that rather than the main line, so that trains slowing to enter the Depot do not slow down trains which are close behind.