Very simple techniques really....
The only paints I used for weathering are Humbrol Enamels. The various shades are all a mixture of just two colours, Matt Black 33 and Matt leather 62.
First I airbrush a layer of 50:50 black and leather over the whole loco, I let this dry a little then take most of it off again with cotton buds soaked in thinners (I use white spirit) working in downward strokes to mimic streaking on the loco sides and making sure to leave paint in the recesses.
The roof gets a coat of the same mix, as well as the bogies. Neat black is used for exhaust stains as well as around the fuel tank and wheel bearings. Light patches of neat matt leather are airbrushed onto parts of the bogies to mimic brake dust and I also dust a very light coat over the whole loco which helps tie all the colours together.
I find very light application of paint at high pressure gives a lovely, dusty finish.
The exhaust on 210 was done with Humbrol matt rust 113, silver 11 and matt black 33 and touched up with some rust coloured weathering powders.
Lastly the buffers faces are touched with some Metalcote Gunmetal. I use a an old tin that has almost dried out, it gives a great representation of the thick grease used on the buffers!