Some modelers weather as a matter of construction. Others say, "I want to present my models as new."
Perhaps it's the latter group that I'm targeting.
I don't care if you maintain equipment like D&RGW did with the California Zephyr in its heyday (they ran the entire train, passengers and all, through a wash rack in Denver so the windows would be clean for the daylight passage through the Rockies), in a few miles, it will pick up dust and grime on the trucks.
So, on my garden Santa Fe passenger F7, I weathered the roof and the trucks. Weathering the trucks takes only a few minutes. The details popped out.
I used a good black acrylic paint from Vallejo. Set the loco on the bench with paper toweling under the wheels. With a medium brush, I applied a few dabs around on the side frames. IMMEDIATELY I used the same brush to apply lots water to the area and wash the paint down, allowing gravity to guide the process. Now you see the need for paper towels!
Work slowly up on it, adding a bit of paint and then washing it down. If you overdo it, more water will remove most of the "grime."
I did not take before and after photos. The next best thing, here are photos of two different trucks, one is done and the other is next.