Hi Ian
If you are weathering using the drybrushing effect then I can help you! I don't know if you do know how to do it but this is the way I do it.
Take a flat Humbrol No- 6/8 brush, Dip it into the desired paint and then with a kitchen towel between your thumb and forfinger drag the brush throughit and most of the paint will come off!
Then you can start by flicking the paintbrush over the raised ares of the loco's bogie detail, This method should ensure you do a bit at a time and remember to look at photograhs of the class of loco if you can't find the actual loco you are doing!! (The weathering effect should be the same)
Have a look at my signature picture of my 37 for some idea.
Please if you want any more help or if you would like me to put a picture of one of my loco's then just ask!
Kind regards
Paul