No, you can load individual items, otherwise you would never be able to do the train because of the amount of barn space needed to collect all the items.
Each item has a separate load button and once you press it that item disappears out of your barn/silo and it is gone.
Press all the individual load buttons and the train will leave straight away giving you a part drop and gem reward.
If you don't press all the individual load buttons the train will leave at the designated time. You will only get a 2 gem reward if you had loaded enough items - I think it is 6 for that reward.
It is good that you have not loaded that iron. Become a hoarder of iron to participate in events .....unless they surprise us and stop using iron.