I have used lots of combinations for weathered track. You really can’t go wrong…My favorite is steam power black overcoated with Testor’s rust. I’ve also used rattle cans from OSH with good results along with lots of other paint combos…
I think that the rails can be the same color. What is most different between the mainline and sidings is the ballast color. Ballast color for sidings etc would be darker because it is not maintained as well as the mainline, therefore it would be oily and dirty, thus darker. I use a slightly darker ballast and then apply a black or burnt umber wash on the non-mainline ballast. The ties would also be older, so that means a lighter brown with a few gray ones mixed in, and a few darker ones too.
Poly Scale Railroad Tie Brown for my mains, and Floquil rust for the sidings and spurs. I have noticed, and a few have too, that PS Railroad Tie Brown varies considerably from batch to batch. Assuming that we all do a fairly good job of mixing it before painting the rails, mine is gray-greenish, while others have posted images showing a nice rusty look. Still others have used plain old craft acrylic paints…see Oct’s MR for an example of craft paint rails…looked darking for my taste, but…maybe with the lighting and eyes it is quite realistic.