Most of train make-up derives from the federal law.

Now the crude oil can be very explosive. So the federal law states that there will be buffer cars between locomotives, business cars and any flat car where it may extend beyond the engineered loading parameters. (para-phrased to English)

Coal (to the US Government) isn't a hazardous material item. It can easily be cleaned up with minimal impact on the environment. (It is almost pure carbon.)

Coiled steel, this might be a railroad specific item. On the BNSF current restrictions on loaded coil steel is as follows: "Loaded coiled steel cars are limited to a maximum of 50 cars on any train type."

You may have just saw a local crew or a switch crew more some cars around.

I hope that this helps.