Hi Dave,
Nice job on the weathering. The most important thing for me is that your locos have lost that "shiny toy" appearance and the matt finish, with the dirt, gives them instant "weight."
If you do fancy going further then have a look at some photographs. My own observations suggest that the wheel faces are never black. Even when brand new locos arrive on the dockside with freshly painted black bogie frames, the wheel faces have a rusty finish. To replicate this I use the technique pioneered by Pelle Soleborg (sp!) of painting with a dark/dirty grey-brown then dusting on dirty rust weathering powder.
I don't know about you, but I am always wary of how colour "scales down." For example, pure black never seems to me to look right in N. I tend to give all my bogies (not just locos - all stock) a dusting of dark grey sprayed from above to create a feeling of depth, and to give the impression of the black "catching the light" in upper facing areas. If you have a copy to hand, the MBAs and Class 60 pictured on p105 of this August's MR have had this treatment - see what you think.
Also, even newly painted locos rapidly acquire a patina of track dust on the bogies - as soon as they've travelled just a few miles it appears - and as it's carried by disturbed air it tends to gather on horizontal or forward facing areas first. Weathering powders or a dusting from the airbrush can help here - once again you can assess the effect for yourself on the models mentioned above.
Peronally, while I find the airbrush invaluable, I find weathering powders are brilliant for depicting some of the more localised spills and stains seen on most railway vehicles.
Incidentally, thanks for posting all the images and updates. I really enjoy following your progress and I think you're doing a great job! I too think the second attempt at the river looks even better than the first!
cheers
Ben A.