In the game Trainstation, you can earn gold (money) by sending passanger or mail from your station using your normal train slots. But the better way is using your international slots, sending trains to as many people as you can. If you send 1 train to 8 people and all of them send you a train back, i am sure you will be able to earn quite a lot.
It is important to have high leveled neighbours.
And you should send trains to your neighbours everyday, so you dont lose any of your senders.