I don't know if this is a topic we have discussed before however it seems like a good idea to bring any thoughts, hints and tips on the subject together in one thread.
Several thin coats are much better than one or two thick coats and a priming coat is always a good idea no matter which material is being painted.
It is a good idea to obtain a large card box and create a spray room within it with a turntable upon which you can place the subject. This keeps the paint within a contained area and being able to turn the model rather than you having to work your way around a static model makes airbrushing more manageable.
Why don't a few of you give it a go on an old loco body that is a bit tired?
You may be able to create that loco that you have always wanted but which the manufacturers always seem to overlook!
And of course airbrushing works well for scenic backdrops, buildings and other model railway subjects. You can practice on a cheap Dapol kit before progressing to something more expensive.