I'm not sure about LEGO, but Märklin, a company that specialized in building train models used simple stainless steel since 1982, chances are that LEGO did the same:
- Stainless steel is relatively cheap
- It doesn't oxidize (as the name implies)
- The electrical charateristics aren't good, but you don't need a very good (and expensive) material like copper for something "small" like toy train tracks (Märklin shows that it works)
- Märklin builT the electric "H0" trains since 1935, such an experienced company might have been a inspiring example when choosing a material
Like starblue said, nickel-siver might also be possible. Another indication of this speculation is the German Wikipedia entry wich says:
Die Schienenprofile von Modelleisenbahnen [...] bestehen mitunter aus Neusilber [...]
Translated: "the rail profile of model railways [...] sometimes consist of nickel-silver [...]"