In the real world railroads (turn of the century) used coal ashes and cinder for roadbeds in railyards adark ballast works well for this as with the grimy oily look and the shades a green grass or grey patches of dirt...
pbr Ground Gravel-Ballast A18
PBR city dirt ## (S) a few nice grey dirt textures.
PBR mixed grass 3
PBR forest ground A47., a nice ivy green texture
PBR grass mixed A67., a bit of dark brown and rootsy green
PBR mud 4., small puddle look
RTXT., has a variety of pbr ballast sets
All on the DLS type in the name in a search...
Dave =)