Airbrush for the most part has been my weapon of choice. But on one of the cars I used some weathering chalk.
Why does the site (webpage??) flip photo's on their side. all 3 of these were taking using upright. Two of them display the way they were photographed, one gets flipped.
The cars look good. I don't go much past what you have done. I have seen some really nice heavily weathered cars with rust and other work, but for me I just like them to look like they have been on the rails for a little while.