I almost only use enamels for weathering, a little acrylic, dry brushed, mainly for staining. The only powder I use is black for soot covered roofs
(or should that be the old term rooves ?) and chimneys.
I don't think you can generalize with weathering, different types of stock attain there own grimy features, steam loco's will have different characteristics to a diesel etc.
The one basic feature that affects all stock is the grime that is thrown up from the track, infrastructure and brake dust all combined especially in bad weather.
To represent this I use a 50/50 mix of Humbrol Dark Earth and Matt Black plus a couple of drops of Leather. Using an air brush this is first applied almost as a mist coat to the lower parts and sides of the stock, built up gradually until its at a degree of grime you'r looking for.
For a steam loco the upper surfaces need a different mix, I use the same basic colours as for the lower area but add a little more black plus some Metalcote Gunmetal about 10%. I don't usually measure it out as I've just got used to mixing it by eye as it were.
This again is layed on in very light coats giving a minute or two in between, again until you have the desired look.
Don't worry if it looks too heavy as this is going to be cut back anyway. Cutting back is done with cotton buds dipped in white spirit, the excess dabbed off on kitchen towel.
Start on the top and work round the bodywork, under handrails etc. working down the sides, leave the smoke box alone unless you want a cleaned loco, you will find gradually that the loco looks as if the cleaners have been at work but the grime is still in all the edges and corners that the rags don't reach, edges of boiler bands, around the flare on the dome or any boiler fittings etc.
How much you take off or leave on will depend on your photo reference, don't guess it.
Tops of chimneys and cab roofs, give them a nice wet coat of matt black enamel and while it's still wet dust it with the black powder, I use a fine tea strainer. It'll make a great mess, do it in a kit box lid then you can salvage the excess.
Let it dry and then blow off the loose.
Back to the airbrush with a thin mix of matt black, Metalcote Gunmetal and blow over the top sufaces again, how much you put on depends on the depth of soot you want.
This is only a rough guide to how I weather an engine, there's a lot more that I do to different parts of the model to get the desired effects but explaining it would take all night, a lot of it is suck it and see mode, laying more mist coats cutting back, adding staining and leaks, thinly covering and cutting back again, and then blowing over again......
you get the picture.
It can take two or three days before I'm happy with the finish.
Constant reference to colour photo's is essential to be honest. B/W photo's might give you a hint but really don't show it how it is.
Try and use a photo of the actual type of loco or stock that's being modelled, if not then something that is very similar.
If you care to have a look here
Gallery you'll see some past examples.
Col.