The first three pictures are the ones I felt gave the best results:
First, Rustall #2 solution as suggested by Mike Cougill in his book, Detailing Track
Second and third are a random combinations of Umber and Gray PanPastels plus a drop or two of India Ink turned into a wash with 91% ISO. The darker ones are heaver on the gray PanPastel or India Ink and represent more recently replaced ties
For full disclosure, here are a few other options I tried:
First up is a spray paint approach using a Testor's Graphite Dust -- It ends up being too metallic looking
Next are some paint-pen trials, but despite being labeled brown, have far too much red in them
I think I'll stick with using the Rustall #2 as a base and then come in with some of the washes and randomly distribute a few darker ones to show some age variation.
The ballast is an approximate 2/3, 1/6, 1/6 mix of medium, fine, and coarse sizes of Woodland Scenics Light Gray ballast. Prototype photos below show varying sizes of rock, so I think mixing them while favoring the medium size provides that look. If anything, I probably should up the contribution of the "fine" size. In the future I might try some real rock products from, say, Arizona Rock and Mineral.
Look for a post later in December, hopefully with tie plates (from Jim Lincoln and Monster Model Works), rail and joint bars (still need to order those from Bill Brillinger's Precision Design Company!