Guys, turn to pages 34-35 in the Winter 2007 (most recent) Classic Trains for 4 pictures of Santa Fe steam that all have different characteristics:
1. 3703: dusty tender, dusty drivers and rods/gear.
2. 3708: shiny tender, dusty drivers, shiny rods/gear.
3. 1371: dusty tender, dusty driver, dusty rods, oily gear.
4. 3522: fairly clean/shiny boiler, slightly dusty tender, clean (shiny) rods/gear.
So depending when a photo was taken in the life of a steam loco (ie, Recent shopping/maintenance), the differences in weathering could result from:
2. Recent oiling.
3. Dirt accumulation during/after a long run.
4. Heavy rains (washing off ash).
5. Rust/scale deposits.
Other notes:
1. The oily gear/rods would accumulate a lot of dirt and grime after a run, but they would be clean and oily before a run.
2. Rain creates a streaking effect over the whole boiler and tender.
3. Model Railroader usually features a lot of steam that has a ash blue look. I don't like this - authentic or not. I believe this this is due to relying on straight up grimy black as the base coat instead of a mix (mine mix is from Joe Brown: 60% grimy black and 40% engine black).