I was working on an old and bent, useless piece Track and brushed on a clear Matte varnish, then liberally added some Weathering Powder. I then sprayed on some more of the varnish to seal it, and it seemed to be OK.  I've scratched it with my Finger nail and it didn't harm it. I only intended it to be in a non-functional Diorama, but sadly I never finished it.  

Thanks for that Ourmick.I will try it out on a spare bit of track.I think there must be other people who have the same problem and might like to give it a try. Audrey

I have used two different methods and am pleased with both although they look quite different. 
Method 1: Woodland Scenics Track Weathering Pens. I have only used the "Rust" one, but it works very effectively and can be done to track already laid. If you give it two coats you can barely tell that there's shiny metal underneath, but perhaps you want to be able to see a little of it. 

Method 2: Brown tamiya acrylic paint (whichever shade you fancy) through airbrush. I picked a brown that looked good on the sleepers and sprayed all my track. Because the dark plastic sleepers are different to the metal of the track, the one paint colour actually turned out looking like two different colours on the sleepers and on the track. I was pleasantly surprised! I then dropped some Tamiya Panel line accentuator on either side of the rail on each sleeper which gives a bit of a dirty of shadowed look (and because it's for panel lines, it runs nicely through the plastic wood grain on the sleepers). 

attached photo is of method 2

I have no idea when you posted your above response, but having received 12 individual  e-mails this morning (all in one go – related to every topic I’ve either started or replied to) I can only put the previous lengthy silences down to the pitiful inefficiencies of the MRS / SMS Web Site, and that perhaps one or two “things” have been suddenly sorted and there's been a re-awakening.

Anyway… I just thought that if you didn’t already know about them, then I’d recommend, quite highly, all the VALLEJO ACRYLICS line of Products for “Weathering” effects and, of course,  general everyday Painting.

If you are NOT familiar with Vallejo, please feel free to tell me and I will be happy to enlighten you further.

Has anybody used the Rusty Rail pens that you can buy? I would be interested to know if they work and are worth buying for this job, as I would also like to make my rails look rusty.

Thanks

Woodland scenics track weathering pens simples!

Weathering track! Practice on an old piece of track first. Take photos of track to get the colour right. As i have said Woodland scenic pens for the rail spot on. For the ballast I like precision paint colours thinned down with white spirit and applied with a pipette.Try it on an old piece of track first. Darker shades around platform and signals.i.e. where locos stand. Any questions message me. Have fun.