Definitions are spot-on;) chances are what GA found is slag left over from copper mining/smelting operations in AZ and NM since the Southern Pacific used it for "ballast". It was heavy and rough and I figure the mines gave it to the railroad for free (or even paid them to take it?) just to keep it from piling up.
As far as the iron and metallic luster on the surface that's how the stuff looks. Like gold leaf it's a super thin layer of copper etc and sometimes a thick layer of iron that later rusts. It's not black as coal when new but almost, then later developes the carnival-glass(?) looking surface after it ages.
The "clinker" I'm familiar with is a yellow/tan/brown color and I guess mostly calcium/magnesium-carbonate left over from the coal?
SP was the last big outfit in the US to switch to diesel-electric but was early in the use of oil burners. I guess it had something to do with the oil fields in southern California and the lack of good coal fields? Coal for the coal fired electric plants around here tends to come from Colorado still.
An iron-rock, as soon as you pick the sucker up) will be obvious as anything. ;)
Alvin in AZ