GFC, I have looked for the coal dust as well, on the tracks that run through the gorge. Even when I walked them right after a coal train went by, didn't see much. But I have read similar that it is a problem.
A little searching finds this by BNSF:
"How extensive is the coal dust problem?
The amount of coal dust that escapes from PRB coal trains is surprisingly large. While the amount of
coal dust that escapes from a particular coal car depends on a number of factors, including the
weather, BNSF has done studies indicating that from 500 lbs to a ton of coal can escape from a single
loaded coal car. Other reports have indicated that as much as 3% of the coal loaded into a coal car
can be lost in transit. In many areas, a thick layer of black coal dust can be observed along the railroad
right of way and in between the tracks. Given the high volume of loaded coal trains that move each day
in the PRB, large amounts of coal dust accumulate rapidly along the PRB rail lines."
Ref:
http://www.beyondtoxics.org/wp-content/uploads/2012/07/BNSF-Coal-Dust-FAQs.pdf
Lots of interesting info there from a railroad's perspective.
The coal trains don't bug me as much as the oil and ethanol trains through the gorge, they are much more hazard in a derailment than coal. Had a big problem with that on the Oregon side not too long ago.