This is a note to anyone else who finds this thread, experiencing the same symptoms: i.e. your inserters are not unloading, or in my case loading my train. I'd done everything by the book: straight tracks and only six inserters per car. The problem turned out to be that I was testing the system, and...

@daniel34 Thanks for the reply, I had not noticed that hovering over the output slot shows the required quantities. That works great.

After I select an item to craft in an assembling machine, it displays the set of components needed to craft that item. The improvement I'd like to see is that just under each component icon is the count of them that are required for each item to be crafted.