The weathering isn't bad for a start.
You might want to even it out a bit, there seems to be "smudges" of dust and dirt. Questions one could ask oneself during the weathering process. Why would there be a lot of dirt inboard of the truck, but very little dirt outboard of the truck? Why would the carbody be all dusty, but the trucks not particularly dusty? Not being critical, just posing questions I would ask as I weather my models. What effect am I trying model? Where does that effect live on a car? What causes it?
For example rust is caused by water and water flows straight down the side of a car. Rust streaks look odd if they are at an angle. Normally rust happens at a joint, seam or where something is moved or scraped.
Dust is billowed up from the ground or settles on top. Cars go in both directions, so dust is evenly distributed. Engines on the other hand tend to go in one direction more, so dust is distributed differently. If you look at an F unit, a common weathering pattern is a "bow wave". Dust and dirt coming up on the side from the lead truck and then decreasing along the side.
I think you are on the right track (no pun intended) and just need some more practice, try making a little more even pattern along the side and feathering it up the side. Make the applications very light. Just a little weathering goes a long way. If you apply to little, you can always go back for more. If you apply too much, cutting it back can be difficult to impossible. Once you get the powders down, try some washes. On the flatcar, try dry brushing some browns and greys across the deck (the boards run across the deck, so brush across the deck, not along the length) and then try a wash of a VERY thin black or burnt umber wash to tone it down and bring out the grain and boards. For washes you can use acrylic craft paints thinned with alcohol, or window cleaner (Windex or ArmourAll).