I don't know, I'm new to this. It seems like if you just let the trains wait at the loading/unloading area it would also cause some kind of traffic jam eventually unless you have a bunch of stackers.
Now, to me it sounds like you want to make a system where a train waits for a load. Once one is available it goes an load, then takes it to a empty destination. You do not want trains waiting at either loading or unloading stations.
The problem with this is, as you have discovered, the train will loop between the waiting bay and any other open station on its schedule. The solution is simple. Instead of a parking bay, just build one unloading bay per train. Trains will wait until fully unloaded at "their" bay, then look for a full loading station to fill up at. Generally you want at least one unloading bay or stacker slot for each train that unloads at a given station. If you do not have enough waiting space you risk blocking your main lines and/or deadlock.
If you have trouble providing enough loads and trains need to wait for a load, stackers will hurt more than they help. In this case you simply have too many trains and should either let trains wait at each loading station or use less trains.
For truely useful help with lots of details
start here and keep reading other things in that subforum. Also, try some multi-player games. Plenty of guys there will be happy to show you how to do trains in great detail.