I used the same airbrush when painting my rails. I used the #3 tip with the air pressure set around 22-25 psi. I think the air pressure setting is going to depend on how thin you make the paint before spraying. Once your paint is ready, use a scrap of lumber or paper and test the air pressure setting and the needle setting to ensure your spray flows with no trouble.
I used the same brand of paint, but applied it with a fairly stiff 1/2" brush. The work went very quickly and it was easy to do 20’ or 30’ of track when I only had a short amount of time available, as clean-up took only seconds. Other advantages were: no masking required anywhere, even if you have sceniced areas or structures alongside the track, no overspray or paint dust on the layout or in the layout room - important if you don’t like cleaning track, and finally, much easier to paint the back (normally unseen) side of the rail, especially on an around-the-room type layout. This last point becomes important if you like to take pictures of your layout, as, sooner or later, you’ll discover that you can get some very realistic-looking shots by placing your camera directly on the layout. Sometimes the camera sees things that are unseen from normal viewing angles.[:-^]
In order to take this picture:
The camera was placed on the “river”, to the right of the loco in this picture - the aisle is to the left. [swg]