Since I model a prototype set in the 50's in the US in the western parts where the ballast is/can be gray, I start by painting the c-track gray using Polly Scale SP Lettering Gray. I don't paint between the rails, since I'm later on adding alot of rust and soot between the rails.

Next I paint the rails on my main line with Polly Scale Roof Brown
(sidings I would paint using Polly Scale PS Rust)
Then the ties using Polly Scale Wooden tie brown
As a base for my ballast, I add Woodland Scenics fine gray ballast. I add regular white glue using a brush

then I sprinkle the ballast

making sure I get enough!

Track ready for the next steps
After I've glued the tracks in place using acrylat/latex, I sprinkle the sides with Woodland Scenics medium gray blend ballast (I use quite alot...)

I glue the ballast in place by first wetting it with rubbing alcohol, and then dripping (soaking!) a white glue-water-rubbing alcohol mix on it
In order to make the main line look like it's busy, I've given the rails some dirt.
Using a brush I first added a wash of PS earth diluted with 5 parts water. Then some PS grimy black diluted with 10 part water and finally on the outside a wash of PS rust 1 to 5.