I spent the last hour and a half painting the track. It's ballasted with good old dirt. I went ahead and did the ballast before the paint, and then airbrushed the entire assembly with a gray brown mix of umber, cream and Payne's gray. After that I picked out the rail with raw sienna and oxide red. Finally, I dry brushed with umber and linen. All of these paints are Liquitex soft body acrylics. I find the pigment goes through the airbrush fine providing you have higher than normal pressure.
The key advantage of an airbrush is control over how heavily the paint goes on and where it goes. If I just wanted to apply a layer of paint I might do it with a rattle can. Using an airbrush affords one much finer modulations. Just my 2 cents. Here's the update: