"Nickel Silver" is an alloy of copper, zinc and nickel. It will "rust", that is, oxidize, but is much more resistant to oxidation than iron or steel. The oxidation products are not "rust" colored here, as no iron is present, but do form slowly and will degrade your track conductivity. Hence the need for "Bright Boys" and the discussion of various cleaners and protecting agents (See the Atlas Forum for an in-depth discussion of this topic!).
"Stainless steel" is an alloy of iron and carbon (steel), with sufficient chromium in the mix to help inhibit oxidation.
While the effect is simular, they are not the same substances.
Brass (an alloy of copper and zinc) will oxidize (tarnish) also.
Most of what I've heard about brass track on other forums is not repeatable on a civilized forum like the Gauge!
Moisture and/or acidity speed up the oxidation process, as has been pointed out, so a dehumidifier in your train room is a good idea unless you are lucky enough to have low humidity year round.
If you are outside - good luck!
And look out for acid rain - especially you Eastern Canadians and NE US folk!
- George