I'll put this thread here as all my models are 3 rail, I'm sure most 3RS people weather or have weathered models, and the others who are just absolutely disgusted by weathering our trains.........well, no comment 
First let me say that I'm NOT saying that what I do, or what I use is the correct way, the best way, or the best materials and techniques to use. I do not consider myself an expert, there are many modelers who are much better at this than I, so I just do what I do for fun, after all it is a hobby. I didn't come up with any of this stuff on my own, I'm not trying to take credit for any of it, I'm just answering some questions for fellow modelers who are curious. So feel free to add your 2 cents, comments, suggestions, critiques (I have thick skin), or your own tips and techniques. I've done a lot of "homework" through the years learning from others and then trying different combinations of my own to achieve the "look" I like for my own models. Working for the railroad for the past 17 years also gives me an up close and personal view of all the modern equipment and what happens to it over time. I can honestly say that I have about 3 of "my own" engines that are weathered, because everything I have done in the past I end up selling because I wasn't happy with it (call me anal, though that's probably not politically correct here). I've done a lot of custom weathering and detailing for clients in the past, but no longer have time for that. I have too many of my own irons in the fire so to speak.
So enough blabbering about the boring stuff.....
Just about all the materials I use are seen here. Pretty much self explanatory although I just show a couple bottles of paint. I use various colors obviously but weathered black and grimy black a lot. I do not use acrylic paint in my airbrush, only Floquil and Scale Coat paints, and yes I know Floquil is discontinued and all the health risks and everything under the sun that we could agrue about re airbrushing. Again, this is what I do...it may not be for you.
Isoproypl alcohol has many uses, but notice it's 70%...for thinning, cleaning, washes, etc. I use 91% to strip paint and it normally works just fine.
This is the compressor I use now as I just got it for Christmas from my parents. Before that I used a big Dewalt unit that was loud and annoying to my neighbors at 10 o'clock at night. This thing is quiet, super quiet, and you can carry it anywhere....like over to my parents house to work on the layout. I have several airbrushes and have tried many over the years. The one on the left is a cheap Harbor Freight $20 dual action model that is great for general use, like painting track, finish coats (dull-cote) and bigger jobs like that. The one to the right is a Master brand, dual action, gravity feed for more detail oriented work (like weathering) and runs about $50ish. I've been airbrushing since around 1985, so cleaning them is no big deal to me, 5 minutes with some thinner/cleaner and a few Q-tips....I'm good to go.
This is a dummy unit I just converted to fixed pilots and Kadee couplers so I'll demonstrate with this since I have a weathered unit already done. It's an MTH 2945-3 Dash 8-40CW, Santa Fe Warbonnet of course.
The 1st thing I do to my diesels is weather all the screens, grids, grills, filters etc. Weathered black, grimy black, and sometimes even a mix of roof brown and black. Depends on how big the screens are and whether they are easily accessible, I tape them off and spray them with a diluted wash....lots of thinner and a little paint. You can always make them darker, but it's real hard to make them lighter without re-painting the original color. I use the green FROG tape, gives a nice tight line and won't peel the paint off. Always pull the tape downward at an angle 
The small grills are too hard to get to without taking the model apart, so I just paint them by hand. There is usually enough paint in the lid from shaking the bottle, so just go slow and take your time. I used to do this in HO scale, I have no idea how, now that I've been doing O scale for years now. I wouldn't even want to try doing this in HO scale now, I'd go blind even quicker!
You can see the difference between these 2 models. The one in front has all the grills blacked out, and I did them all by hand. I think it was actually quicker just to hand paint them, rather than mask off a few of the bigger ones. I also have to mention a lesson learned the hard way... While airbrushing the black around the exhaust stack, I decided to go over the radiator screens on a couple that I had already finished hand painting....why, I don't know. Well in the powered units the motor with the tach tape is right there, and guess what...you can read about that in another thread.
The unit in the back is clean, ready for weathering. The middle unit has the blacked out screens/grills, and the unit in front has been weathered and ready for the layout. This particular unit has a light weathering, just enough to make it look like it has some miles on it. Nothing major, just the stuff that shows signs of use first. It got a wash of ink (brown & a little black) applied with a soft brush over the entire unit. More on the truck side frames than anywhere else. The ink is thinned with the 70% isopropyl, but I don't have a formula. I just go by looks, but its very thin...a lot of thinner. Remember you can always go darker by adding more ink/paint, or just do multiple coats.
You can also airbrush a wash of dirt on the entire unit, but brushing it on allows the wash to flow into all the seems, crack, and crevices like dirt really does.
I darkened up this photo to give you a better shot of how the weathering really looks to the naked eye. The bright light will wash out a lot of the details and how much you can really see. You can see the dirt on the side of the cab...remember to always brush up and down....gravity does the work I the real world.
I use a cheap carousel when I spray the entire unit with the finish Dull-Cote or dirt wash. Also helps so you can spray at an upward angle for the running gear and fuel tank. Watch out for the windows....I usually mask them off for lead units, and spray right over them for middle/intermediate units of the consist. I also remove the crew figures and fold the sun visors down for those units in the middle of the consist.
I use a lot of weathering powders as well, Bragdon and AIM. You see here on the bottom of the plow and the fuel tank, I added a little extra dirt. Earth brown and grimy black work well. Remember though, black powder is very dark so use it sparingly. a little goes a long way.
i.e. the same ole' thing again and again....I've made that mistake a couple times.
