That's a great job! What I've discovered in looking at many, many photos in railroad books is that just about anything goes when it comes to weathering. I'd like to suggest that you develop several different approaches as you go forward....acrylics, Bragdon weathering powders, etc. so you don't end up with all your cars looking like they experienced the exact same dust, dirt, elements, chemicals, etc. As I'm telling you this, I'm reminding myself that I've been weathering too many cars with Bragdon weathering powders and it's time to do some with acrylics, alcohol/india ink, etc.
Keep up the good work.
Mondo