Jonte: I basically use the process in Pelle Soeborgs book "Done in a Day"...
Its really easy...did a few loco's all at the same time...
I start by masking all the glass and numbers. Then I spray the whole loco with a diluted acrylic called Deck Tan by Tamiya(thinned with proper tamiya acryilc thinner almost 1:1). I airbrush in light coats not too much and do more from above to emulate the sun weathering more from above. I then spray the trucks with German Grey Tamiya followed by a grimey colour I custom mixed from Hull Red, Brown and German grey. This colour was swatched from a pic of a truck I zoomed up on till it was just a blur then I mixed and matched... The Grey SSW body was dusted with black powders but not evenly, concentrating on the areas the would stay dark.. The latches and details were given a dab of dilutes Flat Black and then swobbed dry to leave a litte shaddow effect in the depressions etc.
I also scratched some of the lettering and numbers to get a worn effect with a 3M scotch pad, be carefull here and they can do damage!!! This was done before painting.
The tanks are done last with a series or light Dust colours and browny earth tones followed by brushed on oil/diesel spills that are airbrushed ove witha diluted Black to fade the edges. If you look at prototypes you will see that after a spill there is a dark definite line but spills frm befor almost catch dust and fade to a grimey colour. Look at Prototypes and study each individual section...Trucks, Tank, Sides, Lettering and Top... These can be done each in their own way... Next time I do one I will take step by steps and publish them here...could be a while though as I have weathered most of my stock..
One ting I must say is don't over do it... My light Deck Tan base is so faint that only once the powders and other colours are on can you see the effect...and you can always add more once its dry but you can't unpaint!!!
Oh, The rust... Take a fine brush, spot rust near panel edges/corners and wear n tear areas with a red/brown acrylic or oil...then while it is still wet drop earth or dark rust powder on to it...allow to dry the lightly brush the excess off in a downward motion, this leaves a great texture.
NOTE brush EVEYTHING down towards the bottom, Never sideways!!...Dirt does not climb up the loco it washes down> the only dirt that does go up is from the wheel sets and this is airbrushed from a low angle to simulate this....and done in the colour of your roadbed!! Use you airbruch in the same way that Mother Nature uses Mud/Grime and Rust and Sun to weather Locos...
Shoo....thats most of it and it'll get you started