I am gradually weathering all my stock to a consistent level. Mostly I use dry brushing and paint on-wipe off techniques using rail match, humbrol and precision paints. Roofs are 'roof dirt(!)' with talc brushed on when nearly dry to create a better texture. Frames and wheels are painted 'frame dirt' (!!) which is then similarly lightly brushed with talc. Rust colour (light and dark) is added to those areas prone to dry corrosion, oily areas is highlight with 'roof dirt' and then 'brake dust' is very lightly brushed over frames to highlight detail. The phrase 'little is more' applies here otherwise colours can be too garish. I also use 'roof dirt' on those areas where muck collects such as door hinges, door stops, roof board brackets and around window frames. This helps to emphasise the details and bring out the best in what are basically very good but originally shiny plastic models. I find that the only detail that needs to be added are carriage destination boards and tail lamps.
Most of this approach results from years of observation looking at the real thing. It is important to understand how and why dirt builds up on vehicles, where it comes from (smoke, brake dust, oil, corrosion etc) and the fact that it gets there because trains move. You will notice that the state of vehicle that have spent years in sidings differs from vehicles that are regularly used. Unused vehicles tend to turn green and rusty whereas used vehicles turn brown and oily!
I use the same approach to track and structures. The colour of track and ballast is just as important.