There was a thread on this topic not long ago. Bottom line is the best way is what you feel comfortable with. There are many ways to go about it. For flex track (as oppossed to handlaid) I paint the ties with a brush, not being to fussy about neatness. An airbrush could certainly be used if you prefer. Some use spray cans. Also, some paint each length prior to laying. Once I've painted the ties I ballast. Then I paint the rail sides with a small brush. I use roof brown, adding rust tones and blacks at random. I let some of the rust colors get on the ballast at the base of the rail. Then add streaks of oil and grease. etc., down the middle of the track and add spills where appropriate. Have fun!