Here's a thought - prior to building ply and rivet track, do you actually clean the foot of the rail?
The reason for asking is that I've just done that, and wondered why I'd never done it before. I'm making up some lengths of 36" curved track for my demo board. I plucked a length of nickel silver rail from the tube where it's kept, and did as I always do - burnish both sides of the rail with a glass fibre brush to get rid of the crud so that it is clean for soldering.
And then I looked at the foot of the rail... The flat surface, that I'm actually hoping the solder paint will bond to, was just as tarnished as the sides. So I whizzed the brush along that as well, and now all is bright and shiny.
Hopefully it improved the strength of the track rather than just having to rely on solder joints to the web of the rail. I thought that I'd mention it, as I can't recall anyone mentioning it specifically before. Of course, you'll now better and point me to a dozen references :-)