Locomotive & rolling stock weathering

Although not a slave to absolute reproduction of particular vehicles in OO scale (i don't count rivets) I do like to reproduce a weathered look to my stock. In my view, what is the point of trying to accurately replicate an imagined or real location if it is going to look spotlessly clean with endless streams of clean rolling stock? If you are going to model a railway model the grime and muck too.
I've been dabbling with various types of weathering techniques for a while and I find that there is no one particular technique that is good for all finishes or applications.
I use washes, cotton buds, sponge, make up applicators, dry brushing, enamels, acrylics, talcum powder, scenic rust, tamiya weathering sets and airbrushing. In fact, I am awaiting the arrival of my new airbrush and compressor, aerosol propellants just won't do.
Take a look at some of my weathered locomotives.