What kind of metal/alloy were on the old LEGO 9V train tracks?
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 [...]"