Cause of black gunk on track?

It's kind of both and it occurs on all brands of track to some degree.  The gunk is a combination of dirt, both on the rail and in the air and oils deposited from the trains.  It's practically a natural occurrence.

Even after cleaning the track, after a few minutes you'll pick up black gunk with a clean paper towel. 

It's more noticeable on Gargraves because of the flatter railhead.

Rusty

I get it too.  Have to clean the wheels of my locos every couple of months, the track every couple of weeks

The HO scale folks switched to metal wheelsets,  I'm told that besides weight and better tracking it reduces gunk buildup. 

I won't worry about my black gunk, then, just keep on cleanin'. 

I thank you for the insight, gentlemen.

It can eventually cause derailing, or tender whistles, or some other operating car not make good enough connection. Clean it off as needed.

Rob