As information, research into the types of coal used by specific railroads and their supply of water, is always helpful on determining weathering characteristics. For some examples:
1) The New York Central RR generally had/used two types of coal, i.e. one "higher grade, higher BTU Bituminous" for steam locomotives assigned to passenger service, and a slightly "lower/less expensive Bituminous" for all other freight power. Thus, NYC passenger locomotives tended to look less "sooty".
2) The New York Central RR had VERY good water throughout their system, plus an EXCELLENT Water Service Dept. to maintain boiler water treatment chemicals. As a result, most NYC main line steam power tended to NOT have all those white/light mineral deposits all over the tops of their locomotives (as compared to, say the Union Pacific steam locomotives).
3) Weathering steam locomotive running gear machinery tends to be a bit of a mystery to many. It is also worth researching the various lubricants used throughout the different machinery. The plain type side rod bearings used a hard stick grease (Texaco Hi-Tex 8, primarily), while the roller bearing side rods used a soft grease, which was used very sparingly. Other parts of the machinery, such as driving box jaws, received Machine Oil from the mechanical lubricator, while both the valves and cylinders were supplied with Valve Oil (sometimes referred to as 'Steam Oil'), which was supplied by another mechanical lubricator and forced into the steam flow to the valve chest and cylinders. All of these different lubricants leave different looking "deposits" throughout the running gear. Thus, there is rarely any "rust" appearing on the running gear of a main line steam locomotive.