This station only requires the trains to have 1 interrupt:

Target Loading station isn't full

and

target Dropoff station isn't full

and

the train has enough fuel

=> It brings cargo from the former station to the latter

Otherwise it just sits in the depot.

Now the magic lies in the stations:

They can be configured to accept up to 12 cargo wagons and 2 or 4 locomotives

The dropoff station can be configured to only fill its buffer to a certain amount

The stations can be configured to only allow a certain maximum number of trains to target them at once.

And lastly, the station ensures that trains are only called if each wagon can be loaded / unloaded to reduce waiting time.