When using real coal or coke, glue it down to the load-top, make double and triple sure with spray adhesive and/or clear coat.  Rub the result by hand when dry, to persuade any susceptible particle to leave the load.

Do not use loose coal in model railroading.  Any splillage into the track and ballast can cause unfindable short circuits.   Coal never forgets to conduct.



Better to use a sandblasting grade, like Black Beauty, I think it is crushed and sifted obsidian, a black glassy rock.  Can be bought in ten-pound sacks, enough for you and nine of your closest coal friends.   Use this for some uniformity.   Mines produce different sizes, as well as a jumble called run-of-mine .... often used as locomotive coal (cheap).

Amazon link:    https://www.amazon.com/BLACK-BEAUTY-Abrasive-Sandblast-Cabinet/dp/B010VLI694/ref=asc_df_B010VLI694/?tag=hyprod-20&linkCode=df0&hvadid=416743149271&hvpos=&hvnetw=g&hvrand=14755862313913882855&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9030559&hvtargid=pla-887936875385&psc=1&tag=&ref=&adgrpid=93604199893&hvpone=&hvptwo=&hvadid=416743149271&hvpos=&hvnetw=g&hvrand=14755862313913882855&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9030559&hvtargid=pla-887936875385  

Geez.  Site address looks like the biggest decal jumble ever.