I don't have an airbrush, hence I used something rather unconventional - dry pastel which proved to be very effective, and in some essence more versatile than an airbrush.
The method was very simple - take small amount of matte varnish and spread it all over the body of the rolling stock/loco. Then use a flat brush, rub it on the block of dry pastel of suitable color (I used earth, white, black and rust), then dab the powder on top of the wet surface. Then move the brush rapidly in vertically and down motion till you get a dusty, matte finish. Repeat this process across all portion of the rolling stock, including the trucks.
The matte varnish and the natural property of pastel will give a very nice dusty finish. The powder also gets inside the cracks and crevices of the rolling stock and truck, just like the real thing and reveals the details of the model very nicely.
Note that the rust is modeled using acrylic paint - a mix of brunt sienna, Indian red and yellow ochre, and then the appropriate places where rusted using a size 00 brush
And here are some before and after shots.
It's surprising how much of extra detail that I can see now in my locos and rolling stock that I could never spot so easily before. I am confident now that weathering is mandatory for all smaller scale - smaller the scale, more important it is!