Regarding weathering, of both trains and scenery, Let me offer a further viewpoint (albeit long-winded )
Everything in life is not new and shiny. Everything is not worn and weathered.
Sometimes, whole communities get a worn-down, weathered, with a forlorn look to them. Take for example, the layout photos provided, from time to time, by the master craftsman Norm Charbonneau. He presents an entire community that is what I would call antique in appearance, every building, and every train. That is what pleases him. It is certainly pleasing and a real joy for many of us to see. His entire presentation is like a concert, unified in its message and statement. IMHO.
There are individual edifices in parts of my real-life community that are well-worn and getting dilapidated, and some of them are right next to or near newer and well-groomed buildings, with some structures in the process of being built, certainly new and un-weathered.
Some of our fellow hobbyists are strict sticklers for attention to detail and fidelity to time period. That interests me somewhat, when I am crafting a neighborhood altogether, and I surely respect it when I see such modeling accomplished by others. It's fun to see. I enjoy such modeling by others thoroughly and applaud heartily. However, for me, much as I like including some weathered buildings in particular neighborhoods, for example....
...as well as some vehicles, I only pay a mild interest to the circa of vehicles within the community of the layout called, "Moon Township, USA" which is my layout. Why?
1. I consider my time spent on and with my layout to be my playtime. I have consistently amused myself by buying whichever vehicles pleased me, or wished I had actually owned in real-life in the past.
2. When children visit our (my wife's and my) layout, we let well-behaved children select a vehicle right off the layout to take home with them as a toy and as reward for their self-control.
This neighborhood...
....I have made largely un-weathered because that is the kind of neighborhood in which I grew up in the Pittsburgh area. Everybody took good care of their homesteads.
However, there were neighborhoods along the entrance roads close to the steel mills and railroads, within a bicycle ride from my well-groomed neighborhood, which were well-weathered and worn...
...and somewhat forlorn looking, so I enjoyed including them in a particular section of my layout because they were something I knew of personally.
FrankM (talk about long-winded!! Sheeesh!!