Cargo Trains only

Hey I’m new to the game and wondered how I could make sure my stations are accepting Cargo Trains. Some of my stations do others only offer RegionalTrain-contracts whilst others deny red contracts in general. Frankly speaking I don’t get the logic behind it so if you could help me out with this one it would be much appreciated. My current goal is to just have 8 cargos running for tier 2 content. Thanks in advance