There are a lot of methods to weather your rails, but they all boil down more or less to the recipe: Paint the rails a rust color. But then there are many ways to do that - here is my preferred method:
Since I hate the overspray from an airbrush which is ALWAYS dispersed across the whole room (am I doing something wrong??? :mrgreen

. I use the good old paint brush. You have a wide choice of paints, but they should adhere well to metal and plastics (when you are using commercial flex track) or wood (when you are using handlaid track on wooden ties). In most cases Acrylics (e.g. Floquil Polly S) are ok, but lacquer paints (e.g. Tamyia lacquer paints) could attack plastics - so be careful and test the paint on a scrap piece of track first.
Living in Europe I prefer Humbrol enamel paint which is widespread here, but not so well known in the USA, I think. I use paint #70 (brick red) for track weathering (and other rusted things).
Pic 1: I take a not too small brush (No. 1-3) and load it with paint which is a diluted somewhat (about 2/3 paint and 1/3 thinner). Then I slop the paint on rather carelessly, not trying to be very exact. Here you see a stretch of PECO flex track mounted on cork strips. The paint covers the rail sides, but also the track nails and a bit of the ties. What I try to do is to leave the center of the ties free. This gives automatically the impression of spilled oil and soot along the track center.
After painting about a meter (3 ft) of track I wipe off the still wet paint on the railhead with a piece of household tissue paper. The railhead should be absolutely color free and shiny - otherwise you'll get contact problems.
Pic 2: Next step is ballasting the track. I use bird sand, a sort of granite sand which I got dirt cheap (a 2 kilogram bag about $1.50) in a local pet store. I fix it with the usal mix of 'wetted water' (i.e. water with a few drops of dishwasher rinsing agent added, or a water/alcohol-mix) and white glue (water:glue about 3:1). But then I'm also adding a shot of black india ink to the water which helps to tone down the clean ballast.
Pic 3 shows a short piece of double track with additional weeds in the ditch between the two tracks.
Hope this helps you a bit!
Ron