Actually, the rust colors themselves look great. What you have to do is think about the physics and chemistry of rust. Rust first attacks areas that water or moisture is allowed to stand. The means seams and rivets first. You will see some rust in these areas in cars a year or two old. Next, the rust will attack areas where the sun fades and thins the paint. The top of the car is usually the first place. You will often see freight cars that have the top pretty rusty but the rest of the car doesn't have much rust. You will see rust colored streaks in the sides of the car as rain carries the dissolved rust down any vertical surface. The problem with your tank car is the rust on top isn't really connected to the rust on the sides and that seam running along the tank car should also have rust streaks staining the side of the car.
It's really easy to overdue rust with paints. I know, I've done it many times.

I only use chalks for rust now since I can control them much better than paint and I can blend the colors to mix the rust with the underlying color of the car. When you spray the car with a Dulcote type sealer, it reduces that apparent effect of the weathering by about half, which is usually about right. With chalks, you can also use a wet rag and just wipe them off and start over. Saves a lot of cars that get in the junk box otherwise.
Give chalks (sometimes called artist's pastels) a try as an alternative to paint. I think you're getting the idea, you just need to make it so the rust isn't the predominant feature of of your tank car.