The unloader you refer to is a bulk storage ore stacker. It just drops the ore into a pit that feeds to the stackers. You can add Iron ore to most hopper cars, by selecting the '?' in the rollingstock flyout and clicking on the hopper in Surveyor. Make sure you get the one with exactly the same upper and lower case letters in the name, as in the unloader.
The train will slow automatically to the correct speed for unloading.
There is a small building in the middle of the unload area, from where you can run a conveyor to a processing building.(not interactive)
The unloaders will only site in two fixed directions, NS is north-south. EW is east-west.
cheers,
Mike