This is a very controversial subject to say the least. Weathering depends on a variety of many factors. It is probably the most personal of any topic or discipline in the hobby. To many it is difficult to deface a $500 or $600 or more locomotive. It took me several years in the hobby to start weathering motive equipment. I starred first with my scratch built structures. Moved on to box cars, then locos. The problem I saw, was either too much, or not enough weathering. Getting it just right was the key.
Studying real trains in all types of situations was the key, and studying work done by the pros, like the "Weathering Shop" and others was helpful.
However, a certain artistic flair/skill is required. If an individual never painted a picture, or drew or dabbled in any type of serious art, it is difficult to obtain perfect results in a realistic looking weathered unit. It requires more than just dabbing on A/I and powders. However, I found once I started and each one I did, the results were better, and better. Practice makes perfect. I went back and re-weathered my originals.
Now it is second nature. But it was not in the beginning . Another point is even in a yard, not everything is filthy dirty, but everything will have some weathering from natural resources. In the mean time, the modeling suppliers have made a fortune on weathering product$ . The bottom line on this is, how much self confidence and how much prototypical does one have, and want to be. If you start, you'll most likely find it hard to stop... But what ever you do', remember rule # 1. You only have to please yourself. Your railroad, do it your way and have fun'.........