I not sure about graduating. There is so much. I am sure I can weather a plastic diesel but I keeping learning new things on rolling stock.
Using Pan Pastels
Using weathering pencils
Using the airbrush to weather
Moving from airbrushing Vallejo Air paints to Tamiya for my base
Hand brushing Vallejo Air as a wash called “wet on wet”
Using panel liners
Finally using oils for weathering
I constantly buy, weather and sell my rolling stock for the layout. I call it ”churning” the roster. That’s one of the joys of model railroading for me. While I am by no means a Rivet Counter I like the cars on my layout to reflect the eras I operate in. Previously 2005 or 2015 with the emphasis now to just operate in 2015. It’s another opportunity to weather and churn the fleet.
Locomotives do not scare me but they do intimidate me.
I don't know that you can "graduate". I've been weathering for 20 years plus, and my techniques have evolved over the years. Mediums come and go or improve. What I use now is different from what I started using. My palette is much the same as Tom's
Airbrush, with Iwata's weathering set. They are watercolors, and very easy to use. Forgiving as well, since you can wash off anything you don't like, or use a wet bristle brush to lighten any place where you got carried away. I put together what I called "Weathering Set B" when I sold that stuff. It was mostly your basic earth tones and some grays for guys in the steam era, and no washes, as some folks prefer to work with full strength paint or dilute it to their preference.
Vallejo Model Air. You have to be careful if you weather with Vallejo. It has a vinyl component, and if you do other things over top of it, like alcohol or ink washes, or work over it with brushed mediums, it can get shiny, and we don't want shiny in our weathering.
Reaper Miniatures paint. This is a line used by figure modelers. It's a latex, and while you can spray it, it is better suited to brushing. They have many colors we can use, and many more in common with the model railroading palette.
Pan Pastels
Rembrandt chalks (stick form)
Prismacolor colored pencils
Watercolor pencils
AK washes
Oil paints
Craft store paints
Panel liners (Tamiya's)
I'm still using Dullcote (I have a couple of bottles of it and Floquil flat in my stash) but Alclad makes a nice flat coat as well if you prefer working with solvent based materials. As mentioned I like to add a bit of "Dust" to my flat coat.
Stuff that's obsoleted out for me:
Doc Bragdon's weathering powders. Nice but Pan Pastels are easier to apply.
AIM weathering powders. same deal, replaced by Pan Pastels.
I'm still learning new techniques, from other members here, from the military modelers, you name it.
As to another thread, engines don't intimidate me. I just go to Railfan.net and look for photos. Using a photo as a template is the best way to start out. The two most challenging things for me were the tank cars, and steamers. Steamers don't get dirty like everything else on the rails. They're a little different.