Looks OK to me. Mother Nature does not have specifications for the proper application of dirt & grime to railroad rolling stock or locomotives. I once attended a very well presented clinic on weathering, and the fellow had so many photos that there was literally a prototype for everything. Chances are, if someone tells you you've done it wrong, what you should be hearing is "I don't like it" Keep your giant salt shaker handy for the liberal application of grains of salt to take with whatever these types say. The best advice I am willing to give is: work from photographs of the real thing. As for methods: Use them all. No single method does a thorough job. Some sort of blending techniques, such as you are already doing, works best.
Keep up the good work.