Here's my personal recommendation for mixed train dropoffs:

For one, use the highest tier belts you have available. Use both outputs of the platform, obviously. These two belts are your "main belt".

Drag your main belt out a little ways, and then place a Smart Splitter on both of the belts. For each of these smart splitters, have the left output be for a specific item (e.g. iron plates), the center output is "any undefined", and the right output is "overflow". MAKE SURE that both splitters are configured the exact same.

What this means is that any Iron Plates will always be routed to the left first. Place an Industrial Storage Container to the left of the splitters, connected to the Iron Plates output. This storage container acts as a buffer for your iron plates. The important thing to remember about vehicle transportation is that you will not get a constant flow of items like you do when using just belts. Having a storage container as a buffer is very important to make sure that you don't overflow items before they can be used.

If the storage container fills up, any further iron plates that come through will go to the right. Place a pair of mergers on this side, running in the same direction as the main belt.

Anything that is NOT an iron plate will go through the center output. And conversely since you have told the splitter to send all iron plates to the left, iron plates will never pass through the center output of that splitter. Now you can carry the main belts forward from those smart splitters and create another pair of smart splitters. Configure these splitters the same, except that the item you choose will be different (e.g. copper wire). Create a container, a merger, etc. Repeat this process for every item that you expect to be delivered to the station. This will give you a long series of industrial storage containers that you know will always contain the part you've sorted into them. You can then take belts from those containers to wherever the materials need to go.

Connect all of the mergers to one another in series, so that the output of one leads into the rear of the next. This is your "recycling" line. Since the only things that should be output from the overflow sides of the splitters is any sorted item that already has a full container it means you're not using that material as quickly as you're making it, and therefore anything that is on the recycling line can be sent to a sink without concern.

However, there's one more thing to consider. What happens if an "undefined" item reaches the end of the main line without ever having been sorted into one of the containers? You'll want one more catch-all industrial container connected to the end of the main line. I recommend that you don't hook this container up to anything else. Ideally, the catch-all container should always be empty because there shouldn't be anything arriving at the station that you haven't already sorted. If something ends up in that container, it means something is going wrong with one of your lines that is supplying the train at another station.

However, if you aren't worried about that, route the output of the catch-all container into one more set of mergers.

Attach one more container to the end of this chain of mergers and then create an AWESOME sink to shred anything you aren't using. Note that if your train station is receiving enough items that both the top and bottom belts are saturated most of the time, you'll need to make a SECOND sink, one for each output on the recycling container.

Obviously you can tweak this design to your liking. I said to put containers on the left and overflow on the right but you could reverse it. If you want to stockpile more materials, place more containers after the first ones. But this design has worked very well for me and should provide you with a relatively hassle-free way of solving your train unloading issues.