I have started to work on my mining scene. Unlike my previous layout which had an open pit, this will have a head frame to hoist massive sulphides. The small ore cares will be loaded there and then taken back to the plant for processing. The design is based on the Chisel Lake Mine head frame in northern Manitoba. 

i have not been able to find any rule on why some 40 ft containers are only one high, even in a well car.

tried looking under dot, and icc rules. even tried looking for the rail roads rules.

where should i be looking? what language (aka loaded products that is) should i be using?

There are often discussions about how many system cars or private cars or foreign cars one should have on the layout, what's a "proper" mix.

I got access to some data from one day in the summer of 2008 on a generally east-west class one railroad in the upper Midwest.  I did some analysis to see how the cars broke out by system, private or foreign.  I was also able to figure out general car type.  There were about 141 trains in the measurement, with about 14,600 cars over a 24 hour period.  Here is the breakdown of trains:

While visiting family in Spring, TX, I saw these hazmat training cars in the wye.  They are used to train first responders on tank cars and tank car safety, and familiarize them with the various access hatches and connections on tank cars.  While these are very rare cars, they do move around a lot and can be seen in locations that have a lot of hazmat passing through. About 5 years ago I was in Essington, PA (south of Philadelphia) and saw the NS versions of these cars.