The only "real" way of successfully weathering track in my completely biased opinion is to do a little at a time. The appearance of track is extremely complex close up: I have the scars to prove how close up I have been at times. (I broke a couple of ribs at Crewe and bashed them back into place near Northwich a week later
. This was a bit too close).
From a moderate distance (say an ordinary height overbridge looking down on the track at about 30 degrees) the whole lot tends to blend together.
One thing to be decided therefore is whether one wants the detailed or the blended look?
There is a whole host of questions in addition to era. Location, season, traffic and even recent weather are three broad areas. Drainage - good, indifferent and/or bad is another factor that I'm always on about.
Location is the big issue. Where is the track? Industrial, urban, suburban, rural. Even rural brings up some surprises. The track can be very green if wind blows seed onto the ballast and humidity allows it to germinate. Salt (usually along coastal tracks) is another factor. One place I knew in Cornwall as a kid had weird desert specialist plants happily growing in the 4foot. The micro climate of a particular cutting was just right for them. How they got there is anyone's guess.
One thing that I am increasingly inclined to do is to mute the colours added over the basic ballast colourS. Up close diesel/oil spill can be pretty black but the way that light refracts tends to edge it into greys. Water on top of oil-fouled ballast can actually reflect a bright blue sky... That should be fun to model!
Will come back with more...