There is a modeler in England - Paul Owens - who developed a method for modeling gunk between the tracks in engine terminals and yards. I've spent the past few months experimenting on scrap pieces of track to replicate his techniques.  Here's how my latest test worked out. I've described the work on each step in the photo captions.

Track was initially spray painted with Krylon Cammo Brown to initially weather the track. This is HO Peco Code 83. The top track is a siding and the bottom track is another siding where engines are stored. Woodland Scenics dirt/earth was applied throughout.

Next step was to apply Pan Pastel black to the ties on both tracks.

Das modeling clay was spread between the ties. This is the 'gunk" between the rails. I used my finger moistened with water to spread the clay but that typically spread it rail to rail. Next time I'm going to use a popsicle stick that's narrower than the gauge to spread the clay.

Once dried overnight the clay was painted with the following acrylics mixed with talcum powder: talcum powder; black, metallic black, burnt umber and a drop of Vallejo gunmetal blue. After the paint dried I lightly brushed on some Monroe Models grimy black powder.

Added a bumper stop, some figures, a little WS vegetation, a Pennsy coal hopper and a LIRR RS-3.