With large trains, they have to schedule around a number of different road crossings. My guess is that they aren't allowed to run at night due to noise regulations. Since rush hour usually ends around 9 am, they probably figure that 10:10 is less inconvenient.

If you can see the locomotive, you should be able to tell which line is running the train (I would guess BNSF or Union Pacific since you are in PDX). You can check their website for contact info to ask about the schedule.

As for what is on freight trains, they are a very efficient transport for bulk materials. I would guess that a lot of materials come into the marine port and get loaded onto trains. Cars, steel, lumber and other building materials, grain, ore, etc. come in on ships and get loaded and shipped out via rail.

The Port of Porland has a pretty neat slide show. You can see the front end loader filling up the freight cars in a few of the pictures. http://www.portofportland.com/T2_Home.aspx